Messages in this thread | | | Date | Wed, 4 Apr 2012 17:41:48 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem |
| |
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.
>> 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. =)
Sure, I understand why you did it this way.
>> 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.
You misunderstood. Again, I can be wrong, but.
Suppose the task mmmaps /dev/mem (for example). This vma doesn't have VM_NORESERVE (but it has VM_IO).
gup() fails correctly with or without FOLL_FORCE, we should fallback to vma_ops->access().
However. With your patch __access_remote_vm() tries gup() without FOLL_FORCE first and wrongly assumes that it fails because it neeeds FOLL_FORCE and we are going to force-cow.
So __account_vma() adds VM_ACCOUNT before (unnecessary) retry, and this is unnecessary too and wrong.
>> 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.
No, I meant another thing. But yes, I think I was wrong, mprotect_fixup() is fine.
>> 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 =)
Interesting ;)
Oleg.
| |