Re: [PATCH] mm: limit filemap_fault readahead to VMA boundaries

[Jan Kara]

On Wed 22-04-26 13:30:47, Matthew Wilcox wrote:
> On Tue, Apr 21, 2026 at 05:56:07PM -0700, Frederick Mayle wrote:
> > When a file mapping covers a strict subset of a file, an access to the
> > mapping can trigger readahead of file pages outside the mapped region.
> > Readahead is meant to prefetch pages likely to be accessed soon, but
> > these pages aren't accessible via the same means, so it fair to say we
> > don't have a good indicator they'll be accessed soon. Take an ELF file
> > for example: An access to the end of a program's read-only segment isn't
> > a sign that nearby file contents will be accessed next (they are likely
> > to be mapped discontiguously, or not at all). The pressure from loading
> > these pages into the cache can evict more useful pages.
> 
> The problem is that we might (for example) use mprotect() to mark a
> portion of the file as being unmodifiable, but nevertheless still want
> to prefetch through it (since it will be read, just not written).  I'm
> sure this solves your problem, but I'm not sure it covers all use cases.

Well, I'm not sure whether all the usecases are covered either but is what
you describe above something you'd expect people to commonly do? In general
sequential reading through mmap seems to be already relatively rare...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR