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Date: Wed, 04 Apr 2012 13:59:03 +0400
From: Konstantin Khlebnikov <khlebnikov@...nvz.org>
To: Oleg Nesterov <oleg@...hat.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem
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...
>
> Can't really comment the patch, this is not my area. Still,
>
>> + down_write(&mm->mmap_sem);
>> + *pvma = vma = find_vma(mm, addr);
>> + if (vma&& vma->vm_start<= addr) {
>> + ret = vma->vm_end - addr;
>> + if ((vma->vm_flags& (VM_ACCOUNT | VM_NORESERVE | VM_SHARED |
>> + VM_HUGETLB | VM_MAYWRITE)) == VM_MAYWRITE) {
>> + if (!security_vm_enough_memory_mm(mm, vma_pages(vma)))
>
> Oooooh, the whole vma. Say, gdb installs the single breakpoint into
> the huge .text mapping...
We cannot split vma right there, this will be really weird. =)
>
> I am not sure, but probably you want to check at least VM_IO/PFNMAP
> as well. We do not want to charge this memory and retry with FOLL_FORCE
> before vm_ops->access(). Say, /dev/mem
No, VM_IO/PFNMAP aren't affect accounting, there is VM_NORESERVE for this.
>
> Hmm. OTOH, if I am right then mprotect_fixup() should be fixed??
mprotect_fixup() does not account area if it already accounted, so all ok.
>
>
> We drop ->mmap_sem... Say, the task does mremap() in between and
> len == 2 * PAGE_SIZE. Then, for example, copy_to_user_page() can
> write to the same page twice. Perhaps not a problem in practice,
> I dunno.
I have an old unfinished patch which implements upgrade_read() for rw-semaphore =)
>
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