From: Alistair Popple <apopple@nvidia.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-mm@kvack.org, david@fromorbit.com, jhubbard@nvidia.com,
rcampbell@nvidia.com, willy@infradead.org, jgg@nvidia.com,
linux-fsdevel@vger.kernel.org, jack@suse.cz, djwong@kernel.org,
hch@lst.de, david@redhat.com, ruansy.fnst@fujitsu.com,
nvdimm@lists.linux.dev, linux-xfs@vger.kernel.org,
linux-ext4@vger.kernel.org, jglisse@redhat.com
Subject: Re: [RFC 04/10] fs/dax: Don't track page mapping/index
Date: Mon, 15 Apr 2024 17:03:48 +1000 [thread overview]
Message-ID: <878r1f2jko.fsf@nvdebian.thelocal> (raw)
In-Reply-To: <66197008db9fc_36222e294b8@dwillia2-xfh.jf.intel.com.notmuch>
Dan Williams <dan.j.williams@intel.com> writes:
> Jan Kara wrote:
>> On Thu 11-04-24 10:57:25, Alistair Popple wrote:
>> > The page->mapping and page->index fields are normally used by the
>> > pagecache and rmap for looking up virtual mappings of pages. FS DAX
>> > implements it's own kind of page cache and rmap look ups so these
>> > fields are unnecessary. They are currently only used to detect
>> > error/warning conditions which should never occur.
>> >
>> > A future change will change the way shared mappings are detected by
>> > doing normal page reference counting instead, so remove the
>> > unnecessary checks.
>> >
>> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
>> ...
>> > -/*
>> > - * When it is called in dax_insert_entry(), the shared flag will indicate that
>> > - * whether this entry is shared by multiple files. If so, set the page->mapping
>> > - * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
>> > - */
>> > -static void dax_associate_entry(void *entry, struct address_space *mapping,
>> > - struct vm_area_struct *vma, unsigned long address, bool shared)
>> > -{
>> > - unsigned long size = dax_entry_size(entry), pfn, index;
>> > - int i = 0;
>> > -
>> > - if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
>> > - return;
>> > -
>> > - index = linear_page_index(vma, address & ~(size - 1));
>> > - for_each_mapped_pfn(entry, pfn) {
>> > - struct page *page = pfn_to_page(pfn);
>> > -
>> > - if (shared) {
>> > - dax_page_share_get(page);
>> > - } else {
>> > - WARN_ON_ONCE(page->mapping);
>> > - page->mapping = mapping;
>> > - page->index = index + i++;
>> > - }
>> > - }
>> > -}
>>
>> Hum, but what about existing uses of folio->mapping and folio->index in
>> fs/dax.c? AFAICT this patch breaks them. What am I missing? How can this
>> ever work?
I did feel I was missing something here as well, but nothing obviously
breaks with this change from a test perspective (ie. ndctl tests, manual
tests). Somehow I missed how this was used in code, but Dan provided
enough of a hint below though so now I see the errors of my ways :-)
> Right, as far as I can see every fsdax filesystem would need to be
> converted to use dax_holder_operations() so that the fs can backfill
> ->mapping and ->index.
Oh, that was the hint I needed. Thanks. So basically it's just used for
memory failure like so:
memory_failure()
-> memory_failure_dev_pagemap()
-> mf_generic_kill_procs()
-> dax_lock_page()
-> mapping = READ_ONCE(page->mapping);
Somehow I had missed that bleatingly obvious usage of page->mapping. I
also couldn't understand how it was important if it was safe for it to
be just randomly overwritten in the shared case.
But I think I understand now - shared fs dax pages are only supported on
xfs and the mapping/index fields aren't used there because xfs provides
it's own look up for memory failure using dax_holder_operations.
I was initially concerned about these cases because I was wondering if
folio subpages could ever get different mappings and the shared case
implied they could. But it seems that's xfs specific and there is a
separate mechanism to deal with looking up ->mapping/index for that. So
I guess we should still be able to safely store this on the folio
head. I will double check and update this change.
next prev parent reply other threads:[~2024-04-15 7:04 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-11 0:57 [RFC 00/10] fs/dax: Fix FS DAX page reference counts Alistair Popple
2024-04-11 0:57 ` [RFC 01/10] mm/gup.c: Remove redundant check for PCI P2PDMA page Alistair Popple
2024-04-11 12:59 ` Jason Gunthorpe
2024-04-11 13:47 ` David Hildenbrand
2024-04-12 1:37 ` Alistair Popple
2024-04-11 0:57 ` [RFC 02/10] mm/hmm: Remove dead check for HugeTLB and FS DAX Alistair Popple
2024-04-11 12:25 ` Jason Gunthorpe
2024-04-11 13:37 ` Peter Xu
2024-04-12 1:28 ` Alistair Popple
2024-04-11 0:57 ` [RFC 03/10] pci/p2pdma: Don't initialise page refcount to one Alistair Popple
2024-04-11 12:29 ` Jason Gunthorpe
2024-04-12 5:40 ` Alistair Popple
2024-04-12 17:20 ` Dan Williams
2024-05-09 21:59 ` Logan Gunthorpe
2024-05-09 23:14 ` Alistair Popple
2024-04-11 0:57 ` [RFC 04/10] fs/dax: Don't track page mapping/index Alistair Popple
2024-04-12 15:22 ` Jan Kara
2024-04-12 17:31 ` Dan Williams
2024-04-15 7:03 ` Alistair Popple [this message]
2024-04-15 20:51 ` Dan Williams
2024-04-16 0:07 ` Alistair Popple
2024-04-16 0:36 ` Dan Williams
2024-04-12 17:21 ` Dan Williams
2024-04-11 0:57 ` [RFC 05/10] fs/dax: Refactor wait for dax idle page Alistair Popple
2024-04-12 14:37 ` Jan Kara
2024-04-13 20:19 ` John Hubbard
2024-04-15 8:41 ` Alistair Popple
2024-04-11 0:57 ` [RFC 06/10] fs/dax: Add dax_page_free callback Alistair Popple
2024-04-11 0:57 ` [RFC 07/10] mm: Allow compound zone device pages Alistair Popple
2024-04-11 12:32 ` Jason Gunthorpe
2024-04-11 14:10 ` Matthew Wilcox
2024-04-12 1:38 ` Alistair Popple
2024-04-11 0:57 ` [RFC 08/10] fs/dax: Properly refcount fs dax pages Alistair Popple
2024-04-11 0:57 ` [RFC 09/10] mm/khugepage.c: Warn if trying to scan devmap pmd Alistair Popple
2024-04-11 13:45 ` David Hildenbrand
2024-04-12 1:34 ` Alistair Popple
2024-04-11 0:57 ` [RFC 10/10] mm: Remove pXX_devmap Alistair Popple
2024-04-11 12:57 ` Jason Gunthorpe
2024-04-11 17:28 ` [RFC 00/10] fs/dax: Fix FS DAX page reference counts Dan Williams
2024-04-11 17:35 ` Jason Gunthorpe
2024-04-11 17:56 ` Dan Williams
2024-04-12 3:54 ` Alistair Popple
2024-04-12 6:55 ` Alistair Popple
2024-04-12 11:53 ` Jason Gunthorpe
2024-04-12 17:32 ` Dan Williams
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