Re: [PATCH v2] mm: readahead: make thp readahead conditional to mmap_miss logic

[Dev Jain <dev.jain@arm.com>]

On 06/10/25 7:24 am, Roman Gushchin wrote:
> Commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
> introduced a special handling for VM_HUGEPAGE mappings: even if the
> readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are
> allocated.
>
> This change causes a significant regression for containers with a
> tight memory.max limit, if VM_HUGEPAGE is widely used. Prior to this
> commit, mmap_miss logic would eventually lead to the readahead
> disablement, effectively reducing the memory pressure in the
> cgroup. With this change the kernel is trying to allocate 1-2 huge
> pages for each fault, no matter if these pages are used or not
> before being evicted, increasing the memory pressure multi-fold.
>
> To fix the regression, let's make the new VM_HUGEPAGE conditional
> to the mmap_miss check, but keep independent from the ra->ra_pages.
> This way the main intention of commit 4687fdbb805a ("mm/filemap:
> Support VM_HUGEPAGE for file mappings") stays intact, but the
> regression is resolved.
>
> The logic behind this changes is simple: even if a user explicitly
> requests using huge pages to back the file mapping (using VM_HUGEPAGE
> flag), under a very strong memory pressure it's better to fall back
> to ordinary pages.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: linux-mm@kvack.org
>
> --
>
> v2: fixed VM_SEQ_READ handling (by Dev Jain)
>

As you said in a previous mail, we definitely need some other way of measuring
memory pressure here. Since your workload is doing madvise(MADV_HUGEPAGE) (which
means it expects to take advantage of the hugepage by accessing it frequently) and still
getting lots of cache misses, I guess it must be due to swapping out due to
tight memory.max. Anyways, the change looks correct to me.

Reviewed-by: Dev Jain <dev.jain@arm.com>