Re: [PATCH mm-hotfixses] Revert "mm: limit filemap_fault readahead to VMA boundaries"

[Matthew Wilcox]

On Fri, Jun 19, 2026 at 10:43:06AM -0700, Suren Baghdasaryan wrote:
> > Yeah, the mmap usage on SVT-AV1 is a little suspicious, but I guess the
> > pattern may come up time and time again for people doing chunked mmap
> > reads. I think we need to take a closer look at this change.
> 
> Frame-by-frame mmapping for a streaming workoad is quite questionable
> IMHO and I don't think we should be optimizing or encouraging such
> usage, but I understand the reasoning for the revert.

If userspace is properly tuned, it doesn't need automatic readahead
at all; it can disable it and issue manual readaheads.  The point of
automatic readahead is to cope with userspace which is doing stupid
things like calling read() for a single byte at a time.

Could it have better defaults?  Maybe!  It's worth a try.  But those
attempts need to have a very low bar for reversion when it turns out
they affect other workloads.