Received-SPF: pass (google.com: domain of linux-kernel+bounces-75882-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Date: Thu, 22 Feb 2024 03:55:52 +0000
From: Matthew Wilcox <willy@infradead.org>
To: "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	akpm@linux-foundation.org, muchun.song@linux.dev,
	Cyril Hrubis <chrubis@suse.cz>, Jan Stancek <jstancek@redhat.com>,
	Petr Vorel <pvorel@suse.cz>, ltp@lists.linux.it
Subject: Re: [PATCH v2 5/5] hugetlb: Allow faults to be handled under the VMA
 lock
Message-ID: <ZdbFyC6HPr6oKrM-@casper.infradead.org>
References: <20240221234732.187629-1-vishal.moola@gmail.com>
 <20240221234732.187629-6-vishal.moola@gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240221234732.187629-6-vishal.moola@gmail.com>

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@gmail.com>

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