Re: [PATCH 10/11] mm/thp: fix page_vma_mapped_walk() if THP mapped by ptes

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Yang Shi <shy828301@gmail.com>,
	Wang Yugui <wangyugui@e16-tech.com>,
	Matthew Wilcox <willy@infradead.org>,
	Alistair Popple <apopple@nvidia.com>,
	Ralph Campbell <rcampbell@nvidia.com>, Zi Yan <ziy@nvidia.com>,
	Peter Xu <peterx@redhat.com>, Will Deacon <will@kernel.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 10/11] mm/thp: fix page_vma_mapped_walk() if THP mapped by ptes
Date: Thu, 10 Jun 2021 12:42:44 +0300	[thread overview]
Message-ID: <20210610094244.ag3gp5pnu3ulhhgu@box.shutemov.name> (raw)
In-Reply-To: <fedb8632-1798-de42-f39e-873551d5bc81@google.com>

On Wed, Jun 09, 2021 at 11:52:37PM -0700, Hugh Dickins wrote:
> Running certain tests with a DEBUG_VM kernel would crash within hours,
> on the total_mapcount BUG() in split_huge_page_to_list(), while trying
> to free up some memory by punching a hole in a shmem huge page: split's
> try_to_unmap() was unable to find all the mappings of the page (which,
> on a !DEBUG_VM kernel, would then keep the huge page pinned in memory).
> 
> Crash dumps showed two tail pages of a shmem huge page remained mapped
> by pte: ptes in a non-huge-aligned vma of a gVisor process, at the end
> of a long unmapped range; and no page table had yet been allocated for
> the head of the huge page to be mapped into.
> 
> Although designed to handle these odd misaligned huge-page-mapped-by-pte
> cases, page_vma_mapped_walk() falls short by returning false prematurely
> when !pmd_present or !pud_present or !p4d_present or !pgd_present: there
> are cases when a huge page may span the boundary, with ptes present in
> the next.
> 
> Restructure page_vma_mapped_walk() as a loop to continue in these cases,
> while keeping its layout much as before. Add a step_forward() helper to
> advance pvmw->address across those boundaries: originally I tried to use
> mm's standard p?d_addr_end() macros, but hit the same crash 512 times
> less often: because of the way redundant levels are folded together,
> but folded differently in different configurations, it was just too
> difficult to use them correctly; and step_forward() is simpler anyway.
> 
> Fixes: ace71a19cec5 ("mm: introduce page_vma_mapped_walk()")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov