Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

[Jason Wang <jasowang@redhat.com>]

On 2019/3/8 上午5:27, Andrea Arcangeli wrote:
> Hello Jerome,
>
> On Thu, Mar 07, 2019 at 03:17:22PM -0500, Jerome Glisse wrote:
>> So for the above the easiest thing is to call set_page_dirty() from
>> the mmu notifier callback. It is always safe to use the non locking
>> variant from such callback. Well it is safe only if the page was
>> map with write permission prior to the callback so here i assume
>> nothing stupid is going on and that you only vmap page with write
>> if they have a CPU pte with write and if not then you force a write
>> page fault.
> So if the GUP doesn't set FOLL_WRITE, set_page_dirty simply shouldn't
> be called in such case. It only ever makes sense if the pte is
> writable.
>
> On a side note, the reason the write bit on the pte enabled avoids the
> need of the _lock suffix is because of the stable page writeback
> guarantees?
>
>> Basicly from mmu notifier callback you have the same right as zap
>> pte has.
> Good point.
>
> Related to this I already was wondering why the set_page_dirty is not
> done in the invalidate. Reading the patch it looks like the dirty is
> marked dirty when the ring wraps around, not in the invalidate, Jeson
> can tell if I misread something there.


Actually not wrapping around,  the pages for used ring was marked as 
dirty after a round of virtqueue processing when we're sure vhost wrote 
something there.

Thanks


>
> For transient data passing through the ring, nobody should care if
> it's lost. It's not user-journaled anyway so it could hit the disk in
> any order. The only reason to flush it to do disk is if there's memory
> pressure (to pageout like a swapout) and in such case it's enough to
> mark it dirty only in the mmu notifier invalidate like you pointed out
> (and only if GUP was called with FOLL_WRITE).
>
>> O_DIRECT can suffer from the same issue but the race window for that
>> is small enough that it is unlikely it ever happened. But for device
> Ok that clarifies things.
>
>> driver that GUP page for hours/days/weeks/months ... obviously the
>> race window is big enough here. It affects many fs (ext4, xfs, ...)
>> in different ways. I think ext4 is the most obvious because of the
>> kernel log trace it leaves behind.
>>
>> Bottom line is for set_page_dirty to be safe you need the following:
>>      lock_page()
>>      page_mkwrite()
>>      set_pte_with_write()
>>      unlock_page()
> I also wondered why ext4 writepage doesn't recreate the bh if they got
> dropped by the VM and page->private is 0. I mean, page->index and
> page->mapping are still there, that's enough info for writepage itself
> to take a slow path and calls page_mkwrite to find where to write the
> page on disk.
>
>> Now when loosing the write permission on the pte you will first get
>> a mmu notifier callback so anyone that abide by mmu notifier is fine
>> as long as they only write to the page if they found a pte with
>> write as it means the above sequence did happen and page is write-
>> able until the mmu notifier callback happens.
>>
>> When you lookup a page into the page cache you still need to call
>> page_mkwrite() before installing a write-able pte.
>>
>> Here for this vmap thing all you need is that the original user
>> pte had the write flag. If you only allow write in the vmap when
>> the original pte had write and you abide by mmu notifier then it
>> is ok to call set_page_dirty from the mmu notifier (but not after).
>>
>> Hence why my suggestion is a special vunmap that call set_page_dirty
>> on the page from the mmu notifier.
> Agreed, that will solve all issues in vhost context with regard to
> set_page_dirty, including the case the memory is backed by VM_SHARED ext4.
>
> Thanks!
> Andrea