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@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Alex Sierra <alex.sierra@xxxxxxx>
---
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.
