Messages in this thread | | | Date | Sat, 7 Apr 2012 19:33:18 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem |
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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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