linux-kernel - Re: Regression caused by commit 4687fdbb805a ("mm/filemap: Support VM

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Message-ID: <87plcwdyjs.fsf@linux.dev>
Date: Fri, 15 Aug 2025 15:12:39 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,  "linux-mm@...ck.org"
 <linux-mm@...ck.org>,  "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>
Subject: Re: Regression caused by commit 4687fdbb805a ("mm/filemap: Support
 VM_HUGEPAGE for file mappings")

Matthew Wilcox <willy@...radead.org> writes:

> On Fri, Aug 15, 2025 at 11:43:25AM -0700, Roman Gushchin wrote:
>> The commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file
>> mappings") causes a regression in our production for containers
>> which are running short on memory. In some cases they are getting
>> stuck for hours in a vicious reclaim cycle. Reverting this commit
>> fixes the problem.
>> 
>> As I understand, the intention of the commit is to allocate large folios
>> whenever possible, and the idea is to ignore device-specific readahead
>> settings and the mmap_miss logic to achieve that, which makes total
>> sense.
>> 
>> However under a heavy memory pressure there must be a mechanism to
>> revert to order-0 folios, otherwise the memory pressure is inevitable
>> increased. Maybe mmap_miss heuristics should still be applied? Any other
>> ideas how to fix it?
>
> What's supposed to happen is that we should have logic like:
>
>                         if (order > min_order)
>                                 alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
>
> so we try a little bit to free memory if we can't allocate an order-9
> folio immediately, but we shouldn't be retrying for hours.  Maybe
> that got lost somewhere along the line because I don't see it now.

Yeah, I see it in __filemap_get_folio(), but not in ra_alloc_folio().
I'll prepare a fix for this.

>
>> Also, a side question: I wonder if it makes sense to allocate 1-2
>> PMD-sized folios if mapping_large_folio_support() is not there?
>
> Um, we don't?
>
>         if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size)
>                 goto fallback;

Sorry, I wasn't clear, I mean we're still allocating 2-4MB of readahead.
Shouldn't we do something like this instead?

--

diff --git a/mm/filemap.c b/mm/filemap.c
index 983ba1019674..e5fb9034118d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3222,7 +3222,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
        /* Use the readahead code, even if readahead is disabled */
-       if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
+       if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER &&
+           mapping_large_folio_support(mapping)) {
                fpin = maybe_unlock_mmap_for_io(vmf, fpin);
                ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
                ra->size = HPAGE_PMD_NR;