On 12.11.24 10:48, Asahi Lina wrote:
On 11/11/24 8:24 AM, Alistair Popple wrote:
David Hildenbrand <david@xxxxxxxxxx> writes:
On 07.11.24 18:32, Asahi Lina wrote:
On 11/8/24 2:14 AM, David Hildenbrand wrote:
I recall that there is still a problem with false-positives on
folio_test_anon() with ZONE_DEVICE pages, so it's maybe not that
easy ... and the whole get_dev_pagemap() stuff is nasty.
Specifically FS DAX reuses PAGE_MAPPING_ANON in
include/linux/page-flags.h
/*
* Different with flags above, this flag is used only for fsdax mode. It
* indicates that this page->mapping is now under reflink case.
*/
#define PAGE_MAPPING_DAX_SHARED ((void *)0x1)
FS DAX pages are never anon though, so you could probably test for
!vma_is_dax() and/or add an implementation of is_fsdax_page().
Likely we would have to do what GUP does, and temporarily grab a pgmap
reference. Gah.
So if we sort out the pagemap stuff and the possibly wrong
folio_test_anon() on some ZONE_DEVICE pages (but not all, because IIRC
DEVICE_PRIVATE can be anon ...), it might be doable.
Correct, DEVICE_PRIVATE and DEVICE_COHERENT pages are always anon (at
least for now).
But it sounds ugly, especially because that code might change soon and
not require messing with ZONE_DEVICE pages on that level.
Yes, I'm hopoing to get the next version of that series posted this
week. I found a couple of other FS DAX bugs that slowed me down.
- Alistair
And then, we'd not be able to handle VM_PFNMAP cleanly ...
If this is all going to be fixed another way soon then I think there's
no rush to get a workaround in earlier than that, I just don't want it
to fall by the wayside.
We have my original patch downstream in libkrunfw (which despite the
lockdep complaints does work in practice)
I assume it's sufficient to deadlock when a writer pops up after you
succeeded with the first read-locking, and before you start the second
read-locking. IIRC, rwsem is a fair lock, so read-locking when
already-read-locked is not guaranteed to work.
That's why lockdep complains.
--
Cheers,
David / dhildenb
