Re: [ISSUE] Read performance regression when using RWF_DONTCACHE form 8026e49 "mm/filemap: add read support for RWF_DONTCACHE"

[Kiryl Shutsemau]

On Wed, Apr 15, 2026 at 03:28:27PM +0800, Mingyu He wrote:
> The smaller the buffer_size in the test program, the more the
> performance dropped. Initially, I used a 4k buffer_size, and the
> performance decreased significantly. When the buffer_size was
> increased to 128K, the read performance with RWF_DONTCACHE actually
> surpassed the non-flagged version by about 10%.

Maybe you have block size larger than 4k? Core-mm will allocate larger
folios for page cache if filesystem asks it to. And if you try to access
it with 4k buffer it gets multiple read-discard cycles for the same
block with RWF_DONTCACHE. Without RWF_DONTCACHE only the first access to
the block will lead to I/O, following accesses are served from page
cache.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov