linux-kernel - Re: [PATCH v2 5/5] hugetlb: Allow faults to be handled under the VMA lock

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Date: Thu, 22 Feb 2024 03:55:52 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, muchun.song@...ux.dev,
	Cyril Hrubis <chrubis@...e.cz>, Jan Stancek <jstancek@...hat.com>,
	Petr Vorel <pvorel@...e.cz>, ltp@...ts.linux.it
Subject: Re: [PATCH v2 5/5] hugetlb: Allow faults to be handled under the VMA
 lock

On Wed, Feb 21, 2024 at 03:47:32PM -0800, Vishal Moola (Oracle) wrote:
> Hugetlb can now safely handle faults under the VMA lock, so allow it to
> do so.
> 
> This patch may cause ltp hugemmap10 to "fail". Hugemmap10 tests hugetlb
> counters, and expects the counters to remain unchanged on failure to
> handle a fault.
> 
> In hugetlb_no_page(), vmf_anon_prepare() may bailout with no anon_vma
> under the VMA lock after allocating a folio for the hugepage. In
> free_huge_folio(), this folio is completely freed on bailout iff there
> is a surplus of hugetlb pages. This will remove a folio off the freelist
> and decrement the number of hugepages while ltp expects these counters
> to remain unchanged on failure.
> 
> Originally this could only happen due to OOM failures, but now it may
> also occur after we allocate a hugetlb folio without a suitable anon_vma
> under the VMA lock. This should only happen for the first freshly
> allocated hugepage in this vma.

Hmm, so it's a bug in LTP.  Have you talked to the LTP people about it
(cc's added)?

Also, did you try moving the anon_vma check befor the folio allocation
so that you can bail out without disturbing the counters?

> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>