linux-kernel - Re: [PATCH] mm: fix finish_fault() handling for large folios

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Message-ID: <121abab9-5090-486b-a3af-776a9cae04fb@redhat.com>
Date: Wed, 26 Feb 2025 16:42:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, Brian Geffon <bgeffon@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>,
 Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Hugh Dickins
 <hughd@...gle.com>, Marek Maslanka <mmaslanka@...gle.com>
Subject: Re: [PATCH] mm: fix finish_fault() handling for large folios

On 26.02.25 15:03, Matthew Wilcox wrote:
> On Wed, Feb 26, 2025 at 06:48:15AM -0500, Brian Geffon wrote:
>> When handling faults for anon shmem finish_fault() will attempt to install
>> ptes for the entire folio. Unfortunately if it encounters a single
>> non-pte_none entry in that range it will bail, even if the pte that
>> triggered the fault is still pte_none. When this situation happens the
>> fault will be retried endlessly never making forward progress.
>>
>> This patch fixes this behavior and if it detects that a pte in the range
>> is not pte_none it will fall back to setting just the pte for the
>> address that triggered the fault.
> 
> Surely there's a similar problem in do_anonymous_page()?

I recall we handle it in there correctly the last time I stared at it.

We check pte_none to decide which folio size we can allocate (including 
basing the decision on other factors like VMA etc), and after retaking 
the PTL, we recheck vmf_pte_changed / pte_range_none() to make sure 
there were no races.

-- 
Cheers,

David / dhildenb