Re: [PATCH v3 0/8] mm/mprotect: Fix dax puds

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 15, 2024 at 03:21:34PM -0400, Peter Xu wrote:
> [Based on mm-unstable, commit 31334cf98dbd, July 2nd]
> 
> v3:
> - Fix a build issue on i386 PAE config
> - Moved one line from patch 8 to patch 3
> 
> v1: https://lore.kernel.org/r/20240621142504.1940209-1-peterx@redhat.com
> v2: https://lore.kernel.org/r/20240703212918.2417843-1-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.

Ping - any review comments or even tests would be greatly welcomed.

I'm not sure whether this matters for anyone yet so far.  I hope this still
makes sense for DAX even if this is an extremely corner case...

Just to mention the follow up users of this path:

  - huge pfnmap 1G may use this, when VM_PFNMAP can be mapped with 1G too,
    then we should hit similar "bad pud" here.

  - hugetlb rework will use this, when we want this path to process 1G
    hugetlb pages too.

The 1st user is not a must in my initial plan, as VFIO + VM use case
doesn't use mprotect(), so we can keep (1) broken together with DAX 1G
here.  But for the long term we should still fix this, IMHO.

Thanks,

-- 
Peter Xu