Re: [PATCH 2/2] mm: shmem: improve the tmpfs large folio read performance

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/10/18 00:48, Yang Shi wrote:
> On Wed, Oct 16, 2024 at 8:25 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2024/10/17 01:33, Yang Shi wrote:
>>> On Wed, Oct 16, 2024 at 8:38 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>>
>>>> On Wed, Oct 16, 2024 at 06:09:30PM +0800, Baolin Wang wrote:
>>>>> @@ -3128,8 +3127,9 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>>>>>                 if (folio) {
>>>>>                         folio_unlock(folio);
>>>>>
>>>>> -                     page = folio_file_page(folio, index);
>>>>> -                     if (PageHWPoison(page)) {
>>>>> +                     if (folio_test_hwpoison(folio) ||
>>>>> +                         (folio_test_large(folio) &&
>>>>> +                          folio_test_has_hwpoisoned(folio))) {
>>>>
>>>> Hm, so if we have hwpoison set on one page in a folio, we now can't read
>>>> bytes from any page in the folio?  That seems like we've made a bad
>>>> situation worse.
>>>
>>> Yeah, I agree. I think we can fallback to page copy if
>>> folio_test_has_hwpoisoned is true. The PG_hwpoison flag is per page.
>>>
>>> The folio_test_has_hwpoisoned is kept set if the folio split is failed
>>> in memory failure handler.
>>
>> Right. I can still keep the page size copy if
>> folio_test_has_hwpoisoned() is true. Some sample changes are as follow.
>>
>> Moreover, I noticed shmem splice_read() and write() also simply return
>> an error if the folio_test_has_hwpoisoned() is true, without any
>> fallback to page granularity. I wonder if it is worth adding page
>> granularity support as well?
> 
> I think you should do the same.

OK. Let me have a detailed look.

>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 7e79b6a96da0..f30e24e529b9 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3111,9 +3111,11 @@ static ssize_t shmem_file_read_iter(struct kiocb
>> *iocb, struct iov_iter *to)
>>
>>           for (;;) {
>>                   struct folio *folio = NULL;
>> +               struct page *page = NULL;
>>                   unsigned long nr, ret;
>>                   loff_t end_offset, i_size = i_size_read(inode);
>>                   size_t fsize;
>> +               bool fallback_page_copy = false;
>>
>>                   if (unlikely(iocb->ki_pos >= i_size))
>>                           break;
>> @@ -3127,13 +3129,16 @@ static ssize_t shmem_file_read_iter(struct kiocb
>> *iocb, struct iov_iter *to)
>>                   if (folio) {
>>                           folio_unlock(folio);
>>
>> -                       if (folio_test_hwpoison(folio) ||
>> -                           (folio_test_large(folio) &&
>> -                            folio_test_has_hwpoisoned(folio))) {
>> +                       page = folio_file_page(folio, index);
>> +                       if (PageHWPoison(page)) {
>>                                   folio_put(folio);
>>                                   error = -EIO;
>>                                   break;
>>                           }
>> +
>> +                       if (folio_test_large(folio) &&
>> +                           folio_test_has_hwpoisoned(folio))
>> +                               fallback_page_copy = true;
>>                   }
>>
>>                   /*
>> @@ -3147,7 +3152,7 @@ static ssize_t shmem_file_read_iter(struct kiocb
>> *iocb, struct iov_iter *to)
>>                           break;
>>                   }
>>                   end_offset = min_t(loff_t, i_size, iocb->ki_pos +
>> to->count);
>> -               if (folio)
>> +               if (folio && likely(!fallback_page_copy))
>>                           fsize = folio_size(folio);
>>                   else
>>                           fsize = PAGE_SIZE;
>> @@ -3160,8 +3165,13 @@ static ssize_t shmem_file_read_iter(struct kiocb
>> *iocb, struct iov_iter *to)
>>                            * virtual addresses, take care about potential
>> aliasing
>>                            * before reading the page on the kernel side.
>>                            */
>> -                       if (mapping_writably_mapped(mapping))
>> -                               flush_dcache_folio(folio);
>> +                       if (mapping_writably_mapped(mapping)) {
>> +                               if (unlikely(fallback_page_copy))
>> +                                       flush_dcache_page(page);
>> +                               else
>> +                                       flush_dcache_folio(folio);
>> +                       }
>> +
>>                           /*
>>                            * Mark the page accessed if we read the beginning.
>>                            */
>> @@ -3171,7 +3181,10 @@ static ssize_t shmem_file_read_iter(struct kiocb
>> *iocb, struct iov_iter *to)
>>                            * Ok, we have the page, and it's up-to-date, so
>>                            * now we can copy it to user space...
>>                            */
>> -                       ret = copy_folio_to_iter(folio, offset, nr, to);
>> +                       if (unlikely(fallback_page_copy))
>> +                               ret = copy_page_to_iter(page, offset,
>> nr, to);
>> +                       else
>> +                               ret = copy_folio_to_iter(folio, offset,
>> nr, to);
>>                           folio_put(folio);
>>                   } else if (user_backed_iter(to)) {
>>                           /*
> 
> The change seems fine to me.

Thanks.