Re: [PATCH v3 00/25] fs/dax: Fix ZONE_DEVICE page reference counts

David Hildenbrand
2024-12-14T15:23:07Z

On 14.12.24 02:39, Dan Williams wrote:
> [ add akpm and sfr for next steps ]
>
> Alistair Popple wrote:
>> Main updates since v2:
>>
>> - Rename the DAX specific dax_insert_XXX functions to vmf_insert_XXX
>> and have them pass the vmf struct.
>>
>> - Seperate out the device DAX changes.
>>
>> - Restore the page share mapping counting and associated warnings.
>>
>> - Rework truncate to require file-systems to have previously called
>> dax_break_layout() to remove the address space mapping for a
>> page. This found several bugs which are fixed by the first half of
>> the series. The motivation for this was initially to allow the FS
>> DAX page-cache mappings to hold a reference on the page.
>>
>> However that turned out to be a dead-end (see the comments on patch
>> 21), but it found several bugs and I think overall it is an
>> improvement so I have left it here.
>>
>> Device and FS DAX pages have always maintained their own page
>> reference counts without following the normal rules for page reference
>> counting. In particular pages are considered free when the refcount
>> hits one rather than zero and refcounts are not added when mapping the
>> page.
>>
>> Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
>> mechanism for allowing GUP to hold references on the page (see
>> get_dev_pagemap). However there doesn't seem to be any reason why FS
>> DAX pages need their own reference counting scheme.
>>
>> By treating the refcounts on these pages the same way as normal pages
>> we can remove a lot of special checks. In particular pXd_trans_huge()
>> becomes the same as pXd_leaf(), although I haven't made that change
>> here. It also frees up a valuable SW define PTE bit on architectures
>> that have devmap PTE bits defined.
>>
>> It also almost certainly allows further clean-up of the devmap managed
>> functions, but I have left that as a future improvment. It also
>> enables support for compound ZONE_DEVICE pages which is one of my
>> primary motivators for doing this work.
>
> So this is feeling ready for -next exposure, and ideally merged for v6.14. I
> see the comments from John and Bjorn and that you were going to respin for
> that, but if it's just those details things they can probably be handled
> incrementally.
>
> Alistair, are you ready for this to hit -next?
>
> As for which tree...
>
> Andrew, we could take this through -mm, but my first instinct would be to try
> to take it through nvdimm.git mainly to offload any conflict wrangling work and
> small fixups which are likely to be an ongoing trickle.
>
> However, I am not going to put up much of a fight if others prefer this go
> through -mm.
>
> Thoughts?

I'm in the process of preparing v2 of [1] that will result in conflicts
with this series in the rmap code (in particular [PATCH v3 14/25]
huge_memory: Allow mappings of PUD sized pages).

I'll be away for 2 weeks over Christmas, but I assume I'll manage to
post v2 shortly.

Which reminds me that I still have to take a closer look at some things
in this series :) Especially also #14 regarding accounting.

I wonder if we could split out the rmap changes in #14, and have that
patch simply in two trees? No idea.

[1]
https://lore.kernel.org/all/20240829165627.2256514-1-david@redhat.com/T/#u

--
Cheers,

David / dhildenb