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

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Message-ID: <Z79BHbCL3U5aGS0Q@casper.infradead.org>
Date: Wed, 26 Feb 2025 16:28:13 +0000
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: Brian Geffon <bgeffon@...gle.com>,
	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 Wed, Feb 26, 2025 at 04:42:46PM +0100, David Hildenbrand wrote:
> 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.

Ah, so then we'll retry and allocate a folio of the right size the next
time?  Rather than the shmem case where the folio is already allocated
and we can't change that?