[PATCH 1/2] fbdev/defio: Early-out if page is already enlisted

[Thomas Zimmermann]

Hi Sam

Am 10.02.22 um 22:00 schrieb Sam Ravnborg:
> Hi Thomas,
> 
> On Thu, Feb 10, 2022 at 03:11:11PM +0100, Thomas Zimmermann wrote:
>> Return early if a page is already in the list of dirty pages for
>> deferred I/O. This can be detected if the page's list head is not
>> empty. Keep the list head initialized while the page is not enlisted
>> to make this work reliably.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> ---
>>   drivers/video/fbdev/core/fb_defio.c | 21 ++++++++++++---------
>>   1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
>> index a591d291b231..3727b1ca87b1 100644
>> --- a/drivers/video/fbdev/core/fb_defio.c
>> +++ b/drivers/video/fbdev/core/fb_defio.c
>> @@ -59,6 +59,7 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
>>   		printk(KERN_ERR "no mapping available\n");
>>   
>>   	BUG_ON(!page->mapping);
>> +	INIT_LIST_HEAD(&page->lru);
>>   	page->index = vmf->pgoff;
>>   
>>   	vmf->page = page;
>> @@ -122,17 +123,19 @@ static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
>>   	 */
>>   	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 the mkwrite even when the original ps's pte is marked
>> +	 * writable.
>> +	 */
> When moving this comment it would be prudent to also fix the wording a
> bit.
> - Capital in start of sentence and after a period
> - Spell out process and do not shorten ps

Ok.

> 
> 
>> +	if (!list_empty(&page->lru))
>> +		goto page_already_added;
>> +
> 
> This check says that if the page already has something in the parge->lru
> then this is added by defio and thus is already added.
> This matches your commit description - OK.
> 
> Maybe add something like:
> * Pages added will have their lru set, and it is clered again in the
> * deferred work handler.

I'll add a related TODO to the comment because we actually want to 
remove the use of the lru field. It's owned by the page cache. I already 
have a prototype patch that implements the page tracking with a separate 
list.

Best regards
Thomas

> 
> 
> 
>>   	/* we loop through the pagelist before adding in order
>>   	to keep the pagelist sorted */
>>   	list_for_each_entry(cur, &fbdefio->pagelist, lru) {
>> -		/* 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 the
>> -		mkwrite even when the original ps's pte is marked
>> -		writable */
>> -		if (unlikely(cur == page))
>> -			goto page_already_added;
>> -		else if (cur->index > page->index)
>> +		if (cur->index > page->index)
>>   			break;
>>   	}
>>   
>> @@ -194,7 +197,7 @@ static void fb_deferred_io_work(struct work_struct *work)
>>   
>>   	/* clear the list */
>>   	list_for_each_safe(node, next, &fbdefio->pagelist) {
>> -		list_del(node);
>> +		list_del_init(node);
>>   	}
>>   	mutex_unlock(&fbdefio->lock);
>>   }
> 
> With the comment adjusted as you see fit
> Acked-by: Sam Ravnborg <sam at ravnborg.org>

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
-------------- next part --------------
A non-text attachment was scrubbed...
Name: OpenPGP_signature
Type: application/pgp-signature
Size: 840 bytes
Desc: OpenPGP digital signature
URL: <https://lists.freedesktop.org/archives/dri-devel/attachments/20220211/fd633f64/attachment.sig>