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Subject: Re: [PATCH] mm: Skip opportunistic reclaim for dma pinned pages
To:     Jason Gunthorpe <jgg@ziepe.ca>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2020-06-24 16:20, Jason Gunthorpe wrote:
...
> I think Yang explained it - the page is removed from the mappings but
> freeing it does not happen because page_ref_freeze() does not succeed
> due to the pin.
> 
> Presumably the mappings can reconnect to the same physical page if
> it is re-faulted to avoid any data corruption.
> 
> So, the issue here is the mappings are trashed while the page remains
> - and trashing the mapping triggers a mmu notifier which upsets i915.
> 
>> What's less clear is why the comment and the commit description
>> only talk about reclaim, when there are additional things that call
>> try_to_unmap(), including:
>>
>>      migrate_vma_unmap()
>>      split_huge_page_to_list() --> unmap_page()
> 
> It looks like the same unmap first then abort if the refcount is still
> elevated design as shrink_page_list() ?


Yes. I was just wondering why the documentation here seems to ignore the
other, non-reclaim cases. Anyway, though...


> 
>> I do like this code change, though. And I *think* it's actually safe to
>> do this, as it stays away from writeback or other filesystem activity.
>> But let me double check that, in case I'm forgetting something.

...OK, I've checked, and I like it a little bit less now. Mainly for
structural reasons, though. I think it would work correctly. But
here's a concern: try_to_unmap() should only fail to unmap if there is a
reason to not unmap. Having a page be pinned for dma is a reason to not
*free* a page, and it's also a reason to be careful about writeback and
page buffers for writeback and such. But I'm not sure that it's a reason
to fail to remove mappings.

True, most (all?) of the reasons that we remove mappings, generally are
for things that are not allowed while a page is dma-pinned...at least,
today. But still, there's nothing fundamental about a mapping that
should prevent it from coming or going while a page is undergoing
dma.

So, it's merely a convenient, now-misnamed location in the call stack
to fail out. That's not great. It might be better, as Jason hints at
below, to fail out a little earlier, instead. That would lead to a more
places to call page_maybe_dma_pinned(), but that's not a real problem,
because it's still a small number of places.

After writing all of that...I don't feel strongly about it, because
TTU is kind of synonymous with "I'm about to do a dma-pin-unfriendly
operation".

Maybe some of the more experienced fs or mm people have strong opinions
one way or the other?


> 
> It would be nice to have an explanation why it is OK now to change
> it..

Yes. Definitely good to explain that in the commit log. I think
it's triggered by the existence of page_maybe_dma_pinned(). Until
that was added, figuring out if dma was involved required basically
just guesswork. Now we have a way to guess much more accurately. :)

> 
> I don't know, but could it be that try_to_unmap() has to be done
> before checking the refcount as each mapping is included in the
> refcount? ie we couldn't know a DMA pin was active in advance?
> 
> Now that we have your pin stuff we can detect a DMA pin without doing
> all the unmaps?
> 

Once something calls pin_user_page*(), then the pages will be marked
as dma-pinned, yes. So no, there is no need to wait until try_to_unmap()
to find out.

A final note: depending on where page_maybe_dma_pinned() ends up
getting called, this might prevent a fair number of the problems that
Jan originally reported [1], and that I also reported separately!

Well, not all of the problems, and only after the filesystems get
converted to call pin_user_pages() (working on that next), but...I think
it would actually avoid the crash our customer reported back in early
2018. Even though we don't have the full file lease + pin_user_pages()
solution in place.

That's because reclaim is what triggers the problems that we saw. And
with this patch, we bail out of reclaim early.


[1] https://www.spinics.net/lists/linux-mm/msg142700.html


thanks,
-- 
John Hubbard
NVIDIA