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charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20230801124844.278698-3-david@redhat.com> X-Rspam-User: X-Stat-Signature: 8ydpx81uwht7yumwa8a6pgi1njzt1ks6 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: BA640C002A X-HE-Tag: 1690989377-289424 X-HE-Meta: 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 MRYpA5Dc mREJA5+HiPF1sor0y3hWk5fX6jBnugZjG1RjptDZnJXXaQxc0vFg5RtPOg8FAxtY+WGbsjqMSLr1vrmew+6cxN+Yc3QMZ8fC9t5+rkTIRI74eKht++uKtVmT8Yx+UyBua80NM9185jtT9YGHc6p7KvYUtdSvyxnMntZ3FsEC4ClZS93/D/b32BOk6O3FX+KDXuGTHY9w6Oj4kx4B6Z4NPpniHLqvnAfFVHYl7Vxf3yP/r6UyYPMR+kcXVqA2sfrQFy80E X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Aug 01, 2023 at 02:48:38PM +0200, David Hildenbrand wrote: > We shouldn't be using a GUP-internal helper if it can be avoided. > > Similar to smaps_pte_entry() that uses vm_normal_page(), let's use > vm_normal_page_pmd() that similarly refuses to return the huge zeropage. > > In contrast to follow_trans_huge_pmd(), vm_normal_page_pmd(): > > (1) Will always return the head page, not a tail page of a THP. > > If we'd ever call smaps_account with a tail page while setting "compound > = true", we could be in trouble, because smaps_account() would look at > the memmap of unrelated pages. > > If we're unlucky, that memmap does not exist at all. Before we removed > PG_doublemap, we could have triggered something similar as in > commit 24d7275ce279 ("fs/proc: task_mmu.c: don't read mapcount for > migration entry"). > > This can theoretically happen ever since commit ff9f47f6f00c ("mm: proc: > smaps_rollup: do not stall write attempts on mmap_lock"): > > (a) We're in show_smaps_rollup() and processed a VMA > (b) We release the mmap lock in show_smaps_rollup() because it is > contended > (c) We merged that VMA with another VMA > (d) We collapsed a THP in that merged VMA at that position > > If the end address of the original VMA falls into the middle of a THP > area, we would call smap_gather_stats() with a start address that falls > into a PMD-mapped THP. It's probably very rare to trigger when not > really forced. > > (2) Will succeed on a is_pci_p2pdma_page(), like vm_normal_page() > > Treat such PMDs here just like smaps_pte_entry() would treat such PTEs. > If such pages would be anonymous, we most certainly would want to > account them. > > (3) Will skip over pmd_devmap(), like vm_normal_page() for pte_devmap() > > As noted in vm_normal_page(), that is only for handling legacy ZONE_DEVICE > pages. So just like smaps_pte_entry(), we'll now also ignore such PMD > entries. > > Especially, follow_pmd_mask() never ends up calling > follow_trans_huge_pmd() on pmd_devmap(). Instead it calls > follow_devmap_pmd() -- which will fail if neither FOLL_GET nor FOLL_PIN > is set. > > So skipping pmd_devmap() pages seems to be the right thing to do. > > (4) Will properly handle VM_MIXEDMAP/VM_PFNMAP, like vm_normal_page() > > We won't be returning a memmap that should be ignored by core-mm, or > worse, a memmap that does not even exist. Note that while > walk_page_range() will skip VM_PFNMAP mappings, walk_page_vma() won't. > > Most probably this case doesn't currently really happen on the PMD level, > otherwise we'd already be able to trigger kernel crashes when reading > smaps / smaps_rollup. > > So most probably only (1) is relevant in practice as of now, but could only > cause trouble in extreme corner cases. > > Fixes: ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock") > Signed-off-by: David Hildenbrand Maybe move the follow_trans_huge_pmd() declaration from linux/huge_mm.h to mm/internal.h to discourage future mistakes? Otherwise Acked-by: Mel Gorman -- Mel Gorman SUSE Labs