linux-kernel - Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

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Message-ID: <87plqc1tab.fsf@mail.lhotse>
Date: Tue, 13 Aug 2024 22:50:04 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: "Kirill A . Shutemov" <kirill@...temov.name>, Nicholas Piggin
 <npiggin@...il.com>, David Hildenbrand <david@...hat.com>, Matthew Wilcox
 <willy@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, James
 Houghton <jthoughton@...gle.com>, Huang Ying <ying.huang@...el.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>, peterx@...hat.com,
 Vlastimil Babka <vbabka@...e.cz>, Rick P Edgecombe
 <rick.p.edgecombe@...el.com>, Hugh Dickins <hughd@...gle.com>, Borislav
 Petkov <bp@...en8.de>, Christophe Leroy <christophe.leroy@...roup.eu>, Rik
 van Riel <riel@...riel.com>, Dan Williams <dan.j.williams@...el.com>, Mel
 Gorman <mgorman@...hsingularity.net>, x86@...nel.org, Ingo Molnar
 <mingo@...hat.com>, linuxppc-dev@...ts.ozlabs.org, Dave Hansen
 <dave.hansen@...ux.intel.com>, Dave Jiang <dave.jiang@...el.com>, Oscar
 Salvador <osalvador@...e.de>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

Peter Xu <peterx@...hat.com> writes:
> [Based on mm-unstable, commit 98808d08fc0f, Aug 7th. NOTE: it is
>  intentional to not have rebased to latest mm-unstable, as this is to
>  replace the queued v4]
>
> v5 Changelog:
> - Rename patch subject "mm/x86: arch_check_zapped_pud()", add "Implement" [tglx]
> - Mostly rewrote commit messages for the x86 patches, follow -tip rules [tglx]
> - Line wrap fixes (to mostly avoid newlines when unnecessary) [tglx]
> - English fixes [tglx]
> - Fix a build issue only happens with i386 pae + clang
>   https://lore.kernel.org/r/202408111850.Y7rbVXOo-lkp@intel.com
>
> v1: https://lore.kernel.org/r/20240621142504.1940209-1-peterx@redhat.com
> v2: https://lore.kernel.org/r/20240703212918.2417843-1-peterx@redhat.com
> v3: https://lore.kernel.org/r/20240715192142.3241557-1-peterx@redhat.com
> v4: https://lore.kernel.org/r/20240807194812.819412-1-peterx@redhat.com
>
> Dax supports pud pages for a while, but mprotect on puds was missing since
> the start.  This series tries to fix that by providing pud handling in
> mprotect().  The goal is to add more types of pud mappings like hugetlb or
> pfnmaps.  This series paves way for it by fixing known pud entries.
>
> Considering nobody reported this until when I looked at those other types
> of pud mappings, I am thinking maybe it doesn't need to be a fix for stable
> and this may not need to be backported.  I would guess whoever cares about
> mprotect() won't care 1G dax puds yet, vice versa.  I hope fixing that in
> new kernels would be fine, but I'm open to suggestions.
>
> There're a few small things changed to teach mprotect work on PUDs. E.g. it
> will need to start with dropping NUMA_HUGE_PTE_UPDATES which may stop
> making sense when there can be more than one type of huge pte.  OTOH, we'll
> also need to push the mmu notifiers from pmd to pud layers, which might
> need some attention but so far I think it's safe.  For such details, please
> refer to each patch's commit message.
>
> The mprotect() pud process should be straightforward, as I kept it as
> simple as possible.  There's no NUMA handled as dax simply doesn't support
> that.  There's also no userfault involvements as file memory (even if work
> with userfault-wp async mode) will need to split a pud, so pud entry
> doesn't need to yet know userfault's existance (but hugetlb entries will;
> that's also for later).
>
> Tests
> =====
>
> What I did test:
>
> - cross-build tests that I normally cover [1]
>
> - smoke tested on x86_64 the simplest program [2] on dev_dax 1G PUD
>   mprotect() using QEMU's nvdimm emulations [3] and ndctl to create
>   namespaces with proper alignments, which used to throw "bad pud" but now
>   it'll run through all fine.  I checked sigbus happens if with illegal
>   access on protected puds.
>
> - vmtests.
>
> What I didn't test:
>
> - fsdax: I wanted to also give it a shot, but only until then I noticed it
>   doesn't seem to be supported (according to dax_iomap_fault(), which will
>   always fallback on PUD_ORDER).  I did remember it was supported before, I
>   could miss something important there.. please shoot if so.
>
> - userfault wp-async: I also wanted to test userfault-wp async be able to
>   split huge puds (here it's simply a clear_pud.. though), but it won't
>   work for devdax anyway due to not allowed to do smaller than 1G faults in
>   this case. So skip too.
>
> - Power, as no hardware on hand.

Does it need some specific configuration, or just any Power machine will do?

cheers