On 17/07/2019 14:17, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2019-07-17 14:09:00)
On 16/07/2019 16:37, Chris Wilson wrote:
Quoting Tvrtko Ursulin (2019-07-16 16:25:22)
On 16/07/2019 13:49, Chris Wilson wrote:
Following a try_to_unmap() we may want to remove the userptr and so call
put_pages(). However, try_to_unmap() acquires the page lock and so we
must avoid recursively locking the pages ourselves -- which means that
we cannot safely acquire the lock around set_page_dirty(). Since we
can't be sure of the lock, we have to risk skip dirtying the page, or
else risk calling set_page_dirty() without a lock and so risk fs
corruption.
So if trylock randomly fail we get data corruption in whatever data set
application is working on, which is what the original patch was trying
to avoid? Are we able to detect the backing store type so at least we
don't risk skipping set_page_dirty with anonymous/shmemfs?
page->mapping???
Would page->mapping work? What is it telling us?
It basically tells us if there is a fs around; anything that is the most
basic of malloc (even tmpfs/shmemfs has page->mapping).
Normal malloc so anonymous pages? Or you meant everything _apart_ from
the most basic malloc?
We still have the issue that if there is a mapping we should be taking
the lock, and we may have both a mapping and be inside try_to_unmap().
Is this a problem? On a path with mappings we trylock and so solve the
set_dirty_locked and recursive deadlock issues, and with no mappings
with always dirty the page and avoid data corruption.
The problem as I see it is !page->mapping are likely an insignificant
minority of userptr; as I think even memfd are essentially shmemfs (or
hugetlbfs) and so have mappings.
Better then nothing, no? If easy to do..
Regards,
Tvrtko
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