Re: [PATCH] mm: Fix __wp_page_copy_user fallback path for remote mm

[Asahi Lina <lina@asahilina.net>]

On 11/11/24 8:24 AM, Alistair Popple wrote:
> 
> David Hildenbrand <david@redhat.com> 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), so we can keep that in until
the proper solution lands upstream.

Alistair, can you Cc me on future submissions? So I'm reminded to test
this out (FWIW the case I'm interested in is just `gdb /bin/foo` with
the whole root filesystem on DAX, though I imagine a statically linked
foo on a DAX mount by itself is probably an equivalent repro).

~~ Lina