Messages in this thread | | | Date | Sat, 07 Apr 2012 09:11:38 +0400 | From | Konstantin Khlebnikov <> | Subject | Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem |
| |
Hugh Dickins wrote: > On Thu, 5 Apr 2012, Konstantin Khlebnikov wrote: >> Oleg Nesterov wrote: >>> On 04/04, Konstantin Khlebnikov wrote: >>>> Oleg Nesterov wrote: >>>>> On 04/02, Konstantin Khlebnikov wrote: >>>>>> >>>>>> Currently kernel does not account read-only private mappings into >>>>>> memory commitment. >>>>>> But these mappings can be force-COW-ed in get_user_pages(). >>>>> >>>>> Heh. tail -n3 Documentation/vm/overcommit-accounting >>>>> may be you should update it then. >>>> >>>> I just wonder how fragile this accounting... >>> >>> I meant, this patch could also remove this "TODO" from the docs. >> >> Actually I dug into this code for killing VM_ACCOUNT vma flag. >> Currently we cannot do this only because asymmetry in mprotect_fixup(): >> it account vma on read-only -> writable conversion, but keep on backward >> operation. >> Probably we can kill this asymmetry, and after that we can recognize >> accountable vma >> by its others flags state, so we don't need special VM_ACCOUNT for this. > > (I believe the VM_ACCOUNT flag will need to stay.) > > But this is just a quick note to say that I'm not ignoring you: I have > a strong interest in this, but only now found time to look through the > thread and ponder, and I'm not yet ready to decide. > > 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), > but it would be a user-visible change in behaviour, and I've research > to do before arriving at a conclusion.
Agree, but this stuff is very weak. Even if sysctl vm.overcommit_memory=2, probably we should fixup accounting in /proc/pid/mem only for this case, because vm.overcommit_memory=2 supposed to protect against overcommit, but it does not.
> > Hugh
| |