[PATCH v2 2/3] fbdev: Track deferred-I/O pages in pageref struct
Thomas Zimmermann
tzimmermann at suse.de
Tue Apr 26 08:01:05 UTC 2022
Hi
Am 25.04.22 um 20:17 schrieb Sam Ravnborg:
> Hi Thomas,
>
> a little ramblings below. Just my thoughts while trying to understand
> the code - especially since I looked at it before.
Thanks for reviewing the patches.
>
> Sam
>
> On Mon, Apr 25, 2022 at 01:27:50PM +0200, Thomas Zimmermann wrote:
>> Store the per-page state for fbdev's deferred I/O in struct
>> fb_deferred_io_pageref. Maintain a list of pagerefs for the pages
>> that have to be written back to video memory. Update all affected
>> drivers.
>>
>> As with pages before, fbdev acquires a pageref when an mmaped page
>> of the framebuffer is being written to. It holds the pageref in a
>> list of all currently written pagerefs until it flushes the written
>> pages to video memory. Writeback occurs periodically. After writeback
>> fbdev releases all pagerefs and builds up a new dirty list until the
>> next writeback occurs.
>>
>> Using pagerefs has a number of benefits.
>>
>> For pages of the framebuffer, the deferred I/O code used struct
>> page.lru as an entry into the list of dirty pages. The lru field is
>> owned by the page cache, which makes deferred I/O incompatible with
>> some memory pages (e.g., most notably DRM's GEM SHMEM allocator).
>> struct fb_deferred_io_pageref now provides an entry into a list of
>> dirty framebuffer pages, freeing lru for use with the page cache.
>>
>> Drivers also assumed that struct page.index is the page offset into
>> the framebuffer. This is not true for DRM buffers, which are located
>> at various offset within a mapped area. struct fb_deferred_io_pageref
>> explicitly stores an offset into the framebuffer. struct page.index
>> is now only the page offset into the mapped area.
>>
>> These changes will allow DRM to use fbdev deferred I/O without an
>> intermediate shadow buffer.
>>
>> v2:
>> * minor fixes in commit message
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>
>> ---
>
>
>> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c b/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
>> index 02a4a4fa3da3..db02e17e12b6 100644
>> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
>> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
>> @@ -322,12 +322,13 @@ static void vmw_deferred_io(struct fb_info *info,
>> struct vmw_fb_par *par = info->par;
>> unsigned long start, end, min, max;
>> unsigned long flags;
>> - struct page *page;
>> + struct fb_deferred_io_pageref *pageref;
>> int y1, y2;
>>
>> min = ULONG_MAX;
>> max = 0;
>> - list_for_each_entry(page, pagelist, lru) {
>> + list_for_each_entry(pageref, pagelist, list) {
>> + struct page *page = pageref->page;
>> start = page->index << PAGE_SHIFT;
>> end = start + PAGE_SIZE - 1;
>> min = min(min, start);
>
> This is the same change in all deferred_io drivers like above.
> We now traverse pageref->list where pagelist points to the head.
>
> It took me a while to understand that pagelist points to a list of
> fb_deferred_io_pageref and not a list of pages.
> I had been helped, had this been renamed to s/pagelist/pagereflist/.
>
> If you see no point in this then just ignore my comment, I have not
> stared at kernel code for a while and is thus easy to confuse.
It's a good point. There's already a lot going on in this patch and I
wanted to minimize the fallout. But I'll add an additional patch that
fixes the naming.
>
>> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
>> index 6924d489a289..a03b9c64fc61 100644
>> --- a/drivers/video/fbdev/core/fb_defio.c
>> +++ b/drivers/video/fbdev/core/fb_defio.c
>> @@ -36,6 +36,60 @@ static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs
>> return page;
>> }
>>
>> +static struct fb_deferred_io_pageref *fb_deferred_io_pageref_get(struct fb_info *info,
>> + unsigned long offset,
>> + struct page *page)
>> +{
>> + struct fb_deferred_io *fbdefio = info->fbdefio;
>> + struct list_head *pos = &fbdefio->pagelist;
>> + unsigned long pgoff = offset >> PAGE_SHIFT;
>> + struct fb_deferred_io_pageref *pageref, *cur;
>> +
>> + if (WARN_ON_ONCE(pgoff >= info->npagerefs))
>> + return NULL; /* incorrect allocation size */
>> +
>> + /* 1:1 mapping between pageref and page offset */
>> + pageref = &info->pagerefs[pgoff];
>> +
>> + /*
>> + * This check is to catch the case where a new process could start
>> + * writing to the same page through a new PTE. This new access
>> + * can cause a call to .page_mkwrite even if the original process'
>> + * PTE is marked writable.
>> + */
>> + if (!list_empty(&pageref->list))
>> + goto pageref_already_added;
>> +
>> + pageref->page = page;
>> + pageref->offset = pgoff << PAGE_SHIFT;
>> +
>> + if (unlikely(fbdefio->sort_pagelist)) {
>> + /*
>> + * We loop through the list of pagerefs before adding, in
>> + * order to keep the pagerefs sorted. This has significant
>> + * overhead of O(n^2) with n being the number of written
>> + * pages. If possible, drivers should try to work with
>> + * unsorted page lists instead.
>> + */
>> + list_for_each_entry(cur, &info->fbdefio->pagelist, list) {
>> + if (cur > pageref)
>> + break;
>> + }
>> + pos = &cur->list;
>> + }
>> +
>> + list_add_tail(&pageref->list, pos);
>> +
>> +pageref_already_added:
>> + return pageref;
>> +}
>> +
>> +static void fb_deferred_io_pageref_put(struct fb_deferred_io_pageref *pageref,
>> + struct fb_info *info)
>> +{
>> + list_del_init(&pageref->list);
>> +}
>> +
>> /* this is to find and return the vmalloc-ed fb pages */
>> static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>> {
>> @@ -59,7 +113,7 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>> printk(KERN_ERR "no mapping available\n");
>>
>> BUG_ON(!page->mapping);
>> - page->index = vmf->pgoff;
>> + page->index = vmf->pgoff; /* for page_mkclean() */
>>
>> vmf->page = page;
>> return 0;
>> @@ -95,7 +149,11 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
>> struct page *page = vmf->page;
>> struct fb_info *info = vmf->vma->vm_private_data;
>> struct fb_deferred_io *fbdefio = info->fbdefio;
>> - struct list_head *pos = &fbdefio->pagelist;
>> + struct fb_deferred_io_pageref *pageref;
>> + unsigned long offset;
>> + vm_fault_t ret;
>> +
>> + offset = (vmf->address - vmf->vma->vm_start);
>>
>> /* this is a callback we get when userspace first tries to
>> write to the page. we schedule a workqueue. that workqueue
>> @@ -112,6 +170,12 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
>> if (fbdefio->first_io && list_empty(&fbdefio->pagelist))
>> fbdefio->first_io(info);
>>
>> + pageref = fb_deferred_io_pageref_get(info, offset, page);
> Compared to the old code we now do all the sorting and stuff without
> the page locked, which seem like a big change.
We never touch any of the page's fields in fb_deferred_io_pageref_get().
It's only used to initialize the pageref's page pointer. The pagerefs
are all protected by fbdev-internal locking. Is there a reason why we
should further hold the page lock?
All sorting is done by the pageref addresses, which implicitly
correspond to 'offset'. After looking at the new function again, I'll
change it to sort directly by offset. It's clearer in its intend.
>
>
>> + if (WARN_ON_ONCE(!pageref)) {
>> + ret = VM_FAULT_OOM;
>> + goto err_mutex_unlock;
>> + }
>> +
>> /*
>> * We want the page to remain locked from ->page_mkwrite until
>> * the PTE is marked dirty to avoid page_mkclean() being called
>> @@ -120,47 +184,17 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
>> * Do this by locking the page here and informing the caller
>> * about it with VM_FAULT_LOCKED.
>> */
>> - lock_page(page);
>> -
>> - /*
>> - * This check is to catch the case where a new process could start
>> - * writing to the same page through a new PTE. This new access
>> - * can cause a call to .page_mkwrite even if the original process'
>> - * PTE is marked writable.
>> - *
>> - * TODO: The lru field is owned by the page cache; hence the name.
>> - * We dequeue in fb_deferred_io_work() after flushing the
>> - * page's content into video memory. Instead of lru, fbdefio
>> - * should have it's own field.
>> - */
>> - if (!list_empty(&page->lru))
>> - goto page_already_added;
>> -
>> - if (unlikely(fbdefio->sort_pagelist)) {
>> - /*
>> - * We loop through the pagelist before adding in order to
>> - * keep the pagelist sorted. This has significant overhead
>> - * of O(n^2) with n being the number of written pages. If
>> - * possible, drivers should try to work with unsorted page
>> - * lists instead.
>> - */
>> - struct page *cur;
>> -
>> - list_for_each_entry(cur, &fbdefio->pagelist, lru) {
>> - if (cur->index > page->index)
>> - break;
>> - }
>> - pos = &cur->lru;
>> - }
>> -
>> - list_add_tail(&page->lru, pos);
>> + lock_page(pageref->page);
The page lock is taken here.
Best regards
Thomas
>>
>> -page_already_added:
>> mutex_unlock(&fbdefio->lock);
>>
>> /* come back after delay to process the deferred IO */
>> schedule_delayed_work(&info->deferred_work, fbdefio->delay);
>> return VM_FAULT_LOCKED;
>> +
>> +err_mutex_unlock:
>> + mutex_unlock(&fbdefio->lock);
>> + return ret;
>> }
>>
>> static const struct vm_operations_struct fb_deferred_io_vm_ops = {
>> @@ -186,15 +220,14 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_mmap);
>> /* workqueue callback */
>> static void fb_deferred_io_work(struct work_struct *work)
>> {
>> - struct fb_info *info = container_of(work, struct fb_info,
>> - deferred_work.work);
>> - struct list_head *node, *next;
>> - struct page *cur;
>> + struct fb_info *info = container_of(work, struct fb_info, deferred_work.work);
>> + struct fb_deferred_io_pageref *pageref, *next;
>> struct fb_deferred_io *fbdefio = info->fbdefio;
>>
>> /* here we mkclean the pages, then do all deferred IO */
>> mutex_lock(&fbdefio->lock);
>> - list_for_each_entry(cur, &fbdefio->pagelist, lru) {
>> + list_for_each_entry(pageref, &fbdefio->pagelist, list) {
>> + struct page *cur = pageref->page;
>> lock_page(cur);
>> page_mkclean(cur);
>> unlock_page(cur);
>> @@ -204,30 +237,48 @@ static void fb_deferred_io_work(struct work_struct *work)
>> fbdefio->deferred_io(info, &fbdefio->pagelist);
>>
>> /* clear the list */
>> - list_for_each_safe(node, next, &fbdefio->pagelist) {
>> - list_del_init(node);
>> - }
>> + list_for_each_entry_safe(pageref, next, &fbdefio->pagelist, list)
>> + fb_deferred_io_pageref_put(pageref, info);
>> +
>> mutex_unlock(&fbdefio->lock);
>> }
>>
>> -void fb_deferred_io_init(struct fb_info *info)
>> +int fb_deferred_io_init(struct fb_info *info)
>> {
>> struct fb_deferred_io *fbdefio = info->fbdefio;
>> - struct page *page;
>> - unsigned int i;
>> + struct fb_deferred_io_pageref *pagerefs;
>> + unsigned long npagerefs, i;
>> + int ret;
>>
>> BUG_ON(!fbdefio);
>> +
>> + if (WARN_ON(!info->fix.smem_len))
>> + return -EINVAL;
>> +
>> mutex_init(&fbdefio->lock);
>> INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work);
>> INIT_LIST_HEAD(&fbdefio->pagelist);
>> if (fbdefio->delay == 0) /* set a default of 1 s */
>> fbdefio->delay = HZ;
>>
>> - /* initialize all the page lists one time */
>> - for (i = 0; i < info->fix.smem_len; i += PAGE_SIZE) {
>> - page = fb_deferred_io_page(info, i);
>> - INIT_LIST_HEAD(&page->lru);
>> + npagerefs = DIV_ROUND_UP(info->fix.smem_len, PAGE_SIZE);
>> +
>> + /* alloc a page ref for each page of the display memory */
>> + pagerefs = kvcalloc(npagerefs, sizeof(*pagerefs), GFP_KERNEL);
>> + if (!pagerefs) {
>> + ret = -ENOMEM;
>> + goto err;
>> }
>> + for (i = 0; i < npagerefs; ++i)
>> + INIT_LIST_HEAD(&pagerefs[i].list);
>> + info->npagerefs = npagerefs;
>> + info->pagerefs = pagerefs;
>> +
>> + return 0;
>> +
>> +err:
>> + mutex_destroy(&fbdefio->lock);
>> + return ret;
>> }
>> EXPORT_SYMBOL_GPL(fb_deferred_io_init);
>>
>> @@ -254,6 +305,7 @@ void fb_deferred_io_cleanup(struct fb_info *info)
>> page->mapping = NULL;
>> }
>>
>> + kvfree(info->pagerefs);
>> mutex_destroy(&fbdefio->lock);
>> }
>> EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);
>
>> diff --git a/include/linux/fb.h b/include/linux/fb.h
>> index f95da1af9ff6..a332590c0fae 100644
>> --- a/include/linux/fb.h
>> +++ b/include/linux/fb.h
>> @@ -201,6 +201,13 @@ struct fb_pixmap {
>> };
>>
>> #ifdef CONFIG_FB_DEFERRED_IO
>> +struct fb_deferred_io_pageref {
>> + struct page *page;
>> + unsigned long offset;
>> + /* private */
>> + struct list_head list;
> This is the list of pages - so this could be named pagelist...
>
>> +};
>> +
>> struct fb_deferred_io {
>> /* delay between mkwrite and deferred handler */
>> unsigned long delay;
>> @@ -468,6 +475,8 @@ struct fb_info {
>> #endif
>> #ifdef CONFIG_FB_DEFERRED_IO
>> struct delayed_work deferred_work;
>> + unsigned long npagerefs;
>> + struct fb_deferred_io_pageref *pagerefs;
>> struct fb_deferred_io *fbdefio;
>> #endif
>>
>> @@ -661,7 +670,7 @@ static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
>>
>> /* drivers/video/fb_defio.c */
>> int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma);
>> -extern void fb_deferred_io_init(struct fb_info *info);
>> +extern int fb_deferred_io_init(struct fb_info *info);
>> extern void fb_deferred_io_open(struct fb_info *info,
>> struct inode *inode,
>> struct file *file);
>> --
>> 2.36.0
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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