[PATCH v1 0/2] mm/madvise: make MADV_POPULATE_(READ|WRITE) handle VM_FAULT_RETRY properly

From: David Hildenbrand <david@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, David Hildenbrand <david@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Darrick J . Wong" <djwong@kernel.org>,
	John Hubbard <jhubbard@nvidia.com>,
	Jason Gunthorpe <jgg@nvidia.com>, Hugh Dickins <hughd@google.com>
Subject: [PATCH v1 0/2] mm/madvise: make MADV_POPULATE_(READ|WRITE) handle VM_FAULT_RETRY properly
Date: Thu, 14 Mar 2024 17:12:58 +0100	[thread overview]
Message-ID: <20240314161300.382526-1-david@redhat.com> (raw)

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>

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