Re: [RFC PATCH 3/4] mm/shmem: optimize file read with folio batching

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

On 5/19/26 10:15 AM, Chi Zhiling wrote:
> On 5/18/26 14:57, Baolin Wang wrote:
>>
>>
>> On 5/15/26 5:47 PM, Chi Zhiling wrote:
>>> From: Chi Zhiling <chizhiling@kylinos.cn>
>>>
>>> Optimize shmem file read by implementing folio batching in the read
>>> iteration path. Only uptodate folios are added to the batch, ensuring
>>> all folios in the batch are valid and ready for use without additional
>>> checking.
>>>
>>> Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
>>> ---
>>>   mm/shmem.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++++-----
>>>   1 file changed, 93 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> index 767610f78d0d..4bc4e463ca97 100644
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -3348,16 +3348,100 @@ shmem_write_end(const struct kiocb *iocb, 
>>> struct address_space *mapping,
>>>       return copied;
>>>   }
>>> +static pgoff_t shmem_get_read_batch(struct address_space *mapping,
>>> +        pgoff_t index, pgoff_t max, struct folio_batch *fbatch)
>>> +{
>>> +    XA_STATE(xas, &mapping->i_pages, index);
>>> +    struct folio *folio;
>>> +    pgoff_t end = max;
>>> +
>>> +    rcu_read_lock();
>>> +    xas_for_each(&xas, folio, max) {
>>> +        if (xas_retry(&xas, folio))
>>> +            continue;
>>> +        if (xa_is_value(folio)) {
>>> +            end = xas.xa_index;
>>> +            break;
>>> +        }
>>> +        if (!folio_try_get(folio))
>>> +            goto retry;
>>> +
>>> +        if (unlikely(folio != xas_reload(&xas)))
>>> +            goto put_folio;
>>> +
>>> +        end = folio_next_index(folio);
>>> +
>>> +        if (!folio_test_uptodate(folio)) {
>>> +            xas_advance(&xas, end - 1);
>>> +            folio_put(folio);
>>> +            continue;
>>> +        }
>>> +        if (!folio_batch_add(fbatch, folio))
>>> +            break;
>>> +        xas_advance(&xas, end - 1);
>>> +        continue;
>>> +put_folio:
>>> +        folio_put(folio);
>>> +retry:
>>> +        xas_reset(&xas);
>>> +    }
>>> +    rcu_read_unlock();
>>> +
>>> +    return end;
>>> +}
>>
>> I haven't looked at this patch in detail yet, but I feel this part 
>> should belong in mm/filemap.c, or perhaps mm/filemap.c already has a 
>> similar function implemented? Let's wait for Matthew's comments on this.
> 
> You are right. The existing filemap_get_folios_contig can handle this 
> with clearer code, nice!

Good.