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

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Date:	Tue, 3 Apr 2012 16:37:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	linux-mm@...ck.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

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.

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...

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.

Hmm. OTOH, if I am right then mprotect_fixup() should be fixed??

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.

Oleg.

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