Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem

[Oleg Nesterov <oleg@redhat.com>]

On 04/06, Hugh Dickins wrote:
>
> I've long detested that behaviour of GUP write,force, and my strong
> preference would be not to layer more strangeness upon strangeness,
> but limit the damage by making GUP write,force fail in that case,
> instead of inserting a PageAnon page into a VM_SHARED mapping.
>
> I think it's unlikely that it will cause a regression in real life
> (it already fails if you did not open the mmap'ed file for writing),

Yes, and this is what looks confusing to me. Assuming I understand
you (and the code) correctly ;)

If we have a (PROT_READ, MAP_SHARED) file mapping, then FOLL_FORCE
works depending on "file->f_mode & FMODE_WRITE".

Afaics, because do_mmap_pgoff(MAP_SHARED) clears VM_MAYWRITE if
!FMODE_WRITE, and gup(FORCE) checks "vma->vm_flags & VM_MAYWRITE"
before follow_page/etc.

OTOH, if the file was opened without FMODE_WRITE, then I do not
really understand how (PROT_READ, MAP_SHARED) differs from
(PROT_READ, MAP_PRIVATE). However, in the latter case FOLL_FORCE
works, VM_MAYWRITE was not cleared.

Speaking of the difference above, I'd wish I could understand
what VM_MAYSHARE actually means except "MAP_SHARED was used".

Oleg.

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