[REGRESSION] Re: [PATCH v3 3/6] mm: shmem: add large folio support for tmpfs

[Baolin Wang]

Hi,

On 2025/4/30 01:44, Ville Syrjälä wrote:
> On Thu, Nov 28, 2024 at 03:40:41PM +0800, Baolin Wang wrote:
>> Add large folio support for tmpfs write and fallocate paths matching the
>> same high order preference mechanism used in the iomap buffered IO path
>> as used in __filemap_get_folio().
>>
>> Add shmem_mapping_size_orders() to get a hint for the orders of the folio
>> based on the file size which takes care of the mapping requirements.
>>
>> Traditionally, tmpfs only supported PMD-sized large folios. However nowadays
>> with other file systems supporting any sized large folios, and extending
>> anonymous to support mTHP, we should not restrict tmpfs to allocating only
>> PMD-sized large folios, making it more special. Instead, we should allow
>> tmpfs can allocate any sized large folios.
>>
>> Considering that tmpfs already has the 'huge=' option to control the PMD-sized
>> large folios allocation, we can extend the 'huge=' option to allow any sized
>> large folios. The semantics of the 'huge=' mount option are:
>>
>> huge=never: no any sized large folios
>> huge=always: any sized large folios
>> huge=within_size: like 'always' but respect the i_size
>> huge=advise: like 'always' if requested with madvise()
>>
>> Note: for tmpfs mmap() faults, due to the lack of a write size hint, still
>> allocate the PMD-sized huge folios if huge=always/within_size/advise is set.
>>
>> Moreover, the 'deny' and 'force' testing options controlled by
>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', still retain the same
>> semantics. The 'deny' can disable any sized large folios for tmpfs, while
>> the 'force' can enable PMD sized large folios for tmpfs.
>>
>> Co-developed-by: Daniel Gomez <da.gomez at samsung.com>
>> Signed-off-by: Daniel Gomez <da.gomez at samsung.com>
>> Signed-off-by: Baolin Wang <baolin.wang at linux.alibaba.com>
> 
> Hi,
> 
> This causes a huge regression in Intel iGPU texturing performance.

Unfortunately, I don't have such platform to test it.

> 
> I haven't had time to look at this in detail, but presumably the
> problem is that we're no longer getting huge pages from our
> private tmpfs mount (done in i915_gemfs_init()).

IIUC, the i915 driver still limits the maximum write size to PAGE_SIZE 
in the shmem_pwrite(), which prevents tmpfs from allocating large 
folios. As mentioned in the comments below, tmpfs like other file 
systems that support large folios, will allow getting a highest order 
hint based on the size of the write and fallocate paths, and then will 
attempt each allowable huge order.

Therefore, I think the shmem_pwrite() function should be changed to 
remove the limitation that the write size cannot exceed PAGE_SIZE.

Something like the following code (untested):
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c 
b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
index ae3343c81a64..97eefb73c5d2 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
@@ -420,6 +420,7 @@ shmem_pwrite(struct drm_i915_gem_object *obj,
         struct address_space *mapping = obj->base.filp->f_mapping;
         const struct address_space_operations *aops = mapping->a_ops;
         char __user *user_data = u64_to_user_ptr(arg->data_ptr);
+       size_t chunk = mapping_max_folio_size(mapping);
         u64 remain;
         loff_t pos;
         unsigned int pg;
@@ -463,10 +464,10 @@ shmem_pwrite(struct drm_i915_gem_object *obj,
                 void *data, *vaddr;
                 int err;
                 char __maybe_unused c;
+               size_t offset;

-               len = PAGE_SIZE - pg;
-               if (len > remain)
-                       len = remain;
+               offset = pos & (chunk - 1);
+               len = min(chunk - offset, remain);

                 /* Prefault the user page to reduce potential recursion */
                 err = __get_user(c, user_data);