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Date: Sun, 17 Mar 2024 09:50:17 -0700
From: "Darrick J. Wong" <djwong@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Jason Gunthorpe <jgg@nvidia.com>, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v1 0/2] mm/madvise: make MADV_POPULATE_(READ|WRITE)
 handle VM_FAULT_RETRY properly
Message-ID: <20240317165017.GD1927156@frogsfrogsfrogs>
References: <20240314161300.382526-1-david@redhat.com>
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In-Reply-To: <20240314161300.382526-1-david@redhat.com>

On Thu, Mar 14, 2024 at 05:12:58PM +0100, David Hildenbrand wrote:
> Derrick reports that in some cases where pread() would fail with -EIO and
> mmap()+access would generate a SIGBUS signal, MADV_POPULATE_READ /
> MADV_POPULATE_WRITE will keep retrying forever and not fail with -EFAULT.
> 
> It all boils down to missing VM_FAULT_RETRY handling. Let's try to handle
> that in a better way, similar to how ordinary GUP handles it.
> 
> Details in patch #1. In short, move special MADV_POPULATE_(READ|WRITE)
> VMA handling into __get_user_pages(), and make faultin_page_range()
> call __get_user_pages_locked(), which handles VM_FAULT_RETRY. Further,
> avoid the now-useless madvise VMA walk, because __get_user_pages() will
> perform the VMA lookup either way.
> 
> I briefly played with handling the FOLL_MADV_POPULATE checks in
> __get_user_pages() a bit differently, integrating them with existing
> handling, but it ended up looking worse. So I decided to keep it simple.
> 
> Likely, we need better selftests, but the reproducer from Darrick might
> be a bit hard to convert into a simple selftest.
> 
> Note that using mlock() in Darricks reproducer results in a similar
> endless retry. Likely, that is not what we want, and we should handle
> VM_FAULT_RETRY in populate_vma_page_range() / __mm_populate() as well.
> However, similarly using __get_user_pages_locked() might be more
> complicated, because of the advanced VMA handling in
> populate_vma_page_range().
> 
> Further, most populate_vma_page_range() callers simply ignore the return
> values, so it's unclear in which cases we expect to just silently fail, or
> where we'd want to retry+fail or endlessly retry instead.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Darrick J. Wong <djwong@kernel.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Hugh Dickins <hughd@google.com>

After a few days I haven't seen any problems, so
Tested-by: Darrick J. Wong <djwong@kernel.org>

--D

> 
> David Hildenbrand (2):
>   mm/madvise: make MADV_POPULATE_(READ|WRITE) handle VM_FAULT_RETRY
>     properly
>   mm/madvise: don't perform madvise VMA walk for
>     MADV_POPULATE_(READ|WRITE)
> 
>  mm/gup.c      | 54 ++++++++++++++++++++++++++++++---------------------
>  mm/internal.h | 10 ++++++----
>  mm/madvise.c  | 43 +++++++++++++---------------------------
>  3 files changed, 52 insertions(+), 55 deletions(-)
> 
> 
> base-commit: f48159f866f422371bb1aad10eb4d05b29ca4d8c
> -- 
> 2.43.2
>