From: Ralph Campbell <rcampbell@nvidia.com>
To: Felix Kuehling <felix.kuehling@amd.com>,
Alex Sierra <alex.sierra@amd.com>, <akpm@linux-foundation.org>,
<linux-mm@kvack.org>, <linux-ext4@vger.kernel.org>,
<linux-xfs@vger.kernel.org>, "Yang, Philip" <Philip.Yang@amd.com>
Cc: <amd-gfx@lists.freedesktop.org>,
<dri-devel@lists.freedesktop.org>, <hch@lst.de>, <jgg@nvidia.com>,
<jglisse@redhat.com>
Subject: Re: [PATCH v3 2/8] mm: remove extra ZONE_DEVICE struct page refcount
Date: Tue, 29 Jun 2021 17:23:24 -0700 [thread overview]
Message-ID: <4f6dd918-d79b-1aa7-3a4c-caa67ddc29bc@nvidia.com> (raw)
In-Reply-To: <cecd2164-ebfd-382b-af73-992cdc4304b7@amd.com>
On 6/28/21 9:46 AM, Felix Kuehling wrote:
> Am 2021-06-17 um 3:16 p.m. schrieb Ralph Campbell:
>> On 6/17/21 8:16 AM, Alex Sierra wrote:
>>> From: Ralph Campbell <rcampbell@nvidia.com>
>>>
>>> ZONE_DEVICE struct pages have an extra reference count that
>>> complicates the
>>> code for put_page() and several places in the kernel that need to
>>> check the
>>> reference count to see that a page is not being used (gup, compaction,
>>> migration, etc.). Clean up the code so the reference count doesn't
>>> need to
>>> be treated specially for ZONE_DEVICE.
>>>
>>> v2:
>>> AS: merged this patch in linux 5.11 version
>>>
>>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>>> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
>>> ---
>>> arch/powerpc/kvm/book3s_hv_uvmem.c | 2 +-
>>> drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +-
>>> fs/dax.c | 4 +-
>>> include/linux/dax.h | 2 +-
>>> include/linux/memremap.h | 7 +--
>>> include/linux/mm.h | 44 -----------------
>>> lib/test_hmm.c | 2 +-
>>> mm/internal.h | 8 +++
>>> mm/memremap.c | 68 +++++++-------------------
>>> mm/migrate.c | 5 --
>>> mm/page_alloc.c | 3 ++
>>> mm/swap.c | 45 ++---------------
>>> 12 files changed, 45 insertions(+), 147 deletions(-)
>>>
>> I think it is great that you are picking this up and trying to revive it.
>>
>> However, I have a number of concerns about how it affects existing
>> ZONE_DEVICE
>> MEMORY_DEVICE_GENERIC and MEMORY_DEVICE_FS_DAX users and I don't see this
>> addressing them. For example, dev_dax_probe() allocates
>> MEMORY_DEVICE_GENERIC
>> struct pages and then:
>> dev_dax_fault()
>> dev_dax_huge_fault()
>> __dev_dax_pte_fault()
>> vmf_insert_mixed()
>> which just inserts the PFN into the CPU page tables without increasing
>> the page
>> refcount so it is zero (whereas it was one before). But using
>> get_page() will
>> trigger VM_BUG_ON_PAGE() if it is enabled. There isn't any current
>> notion of
>> free verses allocated for these struct pages. I suppose init_page_count()
>> could be called on all the struct pages in dev_dax_probe() to fix that
>> though.
> Hi Ralph,
>
> For DEVICE_GENERIC pages free_zone_device_page doesn't do anything. So
> I'm not sure what the reference counting is good for in this case.
>
> Alex is going to add free_zone_device_page support for DEVICE_GENERIC
> pages (patch 8 of this series). However, even then, it only does
> anything if there is an actual call to put_page. Where would that call
> come from in the dev_dax driver case?
Correct, the drivers/dax/device.c driver allocates MEMORY_DEVICE_GENERIC
struct pages and doesn't seem to allocate/free the page nor increment/decrement
the reference count but it does call vmf_insert_mixed() if the /dev/file
is mmap()'ed into a user process' address space. If devm_memremap_pages()
returns the array of ZONE_DEVICE struct pages initialized with a reference
count of zero, then the CPU page tables will have a PTE/PFN that points to
a struct page with a zero reference count. This goes against the normal
expectation in the rest of the mm code that assumes a page mapped by a CPU
has a non-zero reference count.
So yes, nothing "bad" happens because put_page() isn't called but the
reference count will differ from other drivers that call vmf_insert_mixed()
or vm_insert_page() where the page was allocated with alloc_pages() or
similar.
>> I'm even less clear about how to fix MEMORY_DEVICE_FS_DAX. File
>> systems have clear
>> allocate and free states for backing storage but there are the
>> complications with
>> the page cache references, etc. to consider. The >1 to 1 reference
>> count seems to
>> be used to tell when a page is idle (no I/O, reclaim scanners) rather
>> than free
>> (not allocated to any file) but I'm not 100% sure about that since I
>> don't really
>> understand all the issues around why a file system needs to have a DAX
>> mount option
>> besides knowing that the storage block size has to be a multiple of
>> the page size.
> The only thing that happens in free_zone_device_page for FS_DAX pages is
> wake_up_var(&page->_refcount). I guess, whoever is waiting for this
> wake-up will need to be prepared to see a refcount 0 instead of 1 now. I
> see these callers that would need to be updated:
>
> ./fs/ext4/inode.c: error = ___wait_var_event(&page->_refcount,
> ./fs/ext4/inode.c- atomic_read(&page->_refcount) == 1,
> ./fs/ext4/inode.c- TASK_INTERRUPTIBLE, 0, 0,
> ./fs/ext4/inode.c- ext4_wait_dax_page(ei));
> --
> ./fs/fuse/dax.c: return ___wait_var_event(&page->_refcount,
> ./fs/fuse/dax.c- atomic_read(&page->_refcount) == 1,
> TASK_INTERRUPTIBLE,
> ./fs/fuse/dax.c- 0, 0, fuse_wait_dax_page(inode));
> --
> ./fs/xfs/xfs_file.c: return ___wait_var_event(&page->_refcount,
> ./fs/xfs/xfs_file.c- atomic_read(&page->_refcount) == 1,
> TASK_INTERRUPTIBLE,
> ./fs/xfs/xfs_file.c- 0, 0, xfs_wait_dax_page(inode));
>
> Regarding your page-cache comment, doesn't DAX mean that those file
> pages are not in the page cache?
>
> Regards,
> Felix
>
I don't really understand the FS_DAX code. I can see the __wait_var_event()
is being used when truncating or punching holes in files but I'm not
quite sure if it is using the >1 to 1 reference count to know when a
page has no "extra" references or if it means the page is actually
"free" and no longer assigned to a file.
I really think some FS_DAX expert needs to weigh in on these reference count
changes.
next prev parent reply other threads:[~2021-06-30 0:23 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 15:16 [PATCH v3 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Alex Sierra
2021-06-17 15:16 ` [PATCH v3 1/8] ext4/xfs: add page refcount helper Alex Sierra
2021-06-17 16:11 ` Darrick J. Wong
2021-06-17 23:52 ` Dave Chinner
2021-06-19 16:14 ` Sierra Guiza, Alejandro (Alex)
2021-06-19 18:31 ` Alex Sierra
2021-06-17 15:16 ` [PATCH v3 2/8] mm: remove extra ZONE_DEVICE struct page refcount Alex Sierra
2021-06-17 19:16 ` Ralph Campbell
2021-06-28 16:46 ` Felix Kuehling
2021-06-30 0:23 ` Ralph Campbell [this message]
2021-06-17 15:17 ` [PATCH v3 3/8] kernel: resource: lookup_resource as exported symbol Alex Sierra
2021-06-17 15:17 ` [PATCH v3 4/8] drm/amdkfd: add SPM support for SVM Alex Sierra
2021-06-17 15:17 ` [PATCH v3 5/8] drm/amdkfd: generic type as sys mem on migration to ram Alex Sierra
2021-06-17 15:17 ` [PATCH v3 6/8] include/linux/mm.h: helpers to check zone device generic type Alex Sierra
2021-06-17 15:17 ` [PATCH v3 7/8] mm: add generic type support to migrate_vma helpers Alex Sierra
2021-06-17 15:17 ` [PATCH v3 8/8] mm: call pgmap->ops->page_free for DEVICE_GENERIC pages Alex Sierra
2021-06-17 15:56 ` [PATCH v3 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Sierra Guiza, Alejandro (Alex)
2021-06-20 14:14 ` Theodore Ts'o
2021-06-23 21:49 ` Felix Kuehling
2021-06-24 5:30 ` Christoph Hellwig
2021-06-24 15:08 ` Felix Kuehling
2021-07-16 15:07 ` Theodore Y. Ts'o
2021-07-16 22:14 ` Felix Kuehling
2021-07-17 19:54 ` Sierra Guiza, Alejandro (Alex)
2021-07-23 22:46 ` Sierra Guiza, Alejandro (Alex)
2021-07-30 19:02 ` Felix Kuehling
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