Re: [PATCH v6 21/26] fs/dax: Properly refcount fs dax pages

Dan Williams
2025-01-14T03:35:17Z

Alistair Popple wrote:
> Currently fs dax pages are considered free when the refcount drops to
> one and their refcounts are not increased when mapped via PTEs or
> decreased when unmapped. This requires special logic in mm paths to
> detect that these pages should not be properly refcounted, and to
> detect when the refcount drops to one instead of zero.
>
> On the other hand get_user_pages(), etc. will properly refcount fs dax
> pages by taking a reference and dropping it when the page is
> unpinned.
>
> Tracking this special behaviour requires extra PTE bits
> (eg. pte_devmap) and introduces rules that are potentially confusing
> and specific to FS DAX pages. To fix this, and to possibly allow
> removal of the special PTE bits in future, convert the fs dax page
> refcounts to be zero based and instead take a reference on the page
> each time it is mapped as is currently the case for normal pages.
>
> This may also allow a future clean-up to remove the pgmap refcounting
> that is currently done in mm/gup.c.

This patch depends on FS_DAX_LIMITED being abandoned first, so do
include the patch at the bottom of this reply in your series before this
patch.

> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>
> ---
>
> Changes since v2:
>
> Based on some questions from Dan I attempted to have the FS DAX page
> cache (ie. address space) hold a reference to the folio whilst it was
> mapped. However I came to the strong conclusion that this was not the
> right thing to do.
>
> If the page refcount == 0 it means the page is:
>
> 1. not mapped into user-space
> 2. not subject to other access via DMA/GUP/etc.
>
> Ie. From the core MM perspective the page is not in use.
>
> The fact a page may or may not be present in one or more address space
> mappings is irrelevant for core MM. It just means the page is still in
> use or valid from the file system perspective, and it's a
> responsiblity of the file system to remove these mappings if the pfn
> mapping becomes invalid (along with first making sure the MM state,
> ie. page->refcount, is idle). So we shouldn't be trying to track that
> lifetime with MM refcounts.
>
> Doing so just makes DMA-idle tracking more complex because there is
> now another thing (one or more address spaces) which can hold
> references on a page. And FS DAX can't even keep track of all the
> address spaces which might contain a reference to the page in the
> XFS/reflink case anyway.
>
> We could do this if we made file systems invalidate all address space
> mappings prior to calling dax_break_layouts(), but that isn't
> currently neccessary and would lead to increased faults just so we
> could do some superfluous refcounting which the file system already
> does.
>
> I have however put the page sharing checks and WARN_ON's back which
> also turned out to be useful for figuring out when to re-initialising
> a folio.

I feel like these comments are a useful analysis that deserve not to be
lost to the sands of time on the list.

Perhaps capture a flavor of this relevant for future consideration in a
"DAX page Lifetime" section of Documentation/filesystems/dax.rst?

> ---
> drivers/nvdimm/pmem.c | 4 +-
> fs/dax.c | 212 +++++++++++++++++++++++-----------------
> fs/fuse/virtio_fs.c | 3 +-
> fs/xfs/xfs_inode.c | 2 +-
> include/linux/dax.h | 6 +-
> include/linux/mm.h | 27 +-----
> include/linux/mm_types.h | 7 +-
> mm/gup.c | 9 +--
> mm/huge_memory.c | 6 +-
> mm/internal.h | 2 +-
> mm/memory-failure.c | 6 +-
> mm/memory.c | 6 +-
> mm/memremap.c | 47 ++++-----
> mm/mm_init.c | 9 +--
> mm/swap.c | 2 +-
> 15 files changed, 183 insertions(+), 165 deletions(-)
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index d81faa9..785b2d2 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -513,7 +513,7 @@ static int pmem_attach_disk(struct device *dev,
>
> pmem->disk = disk;
> pmem->pgmap.owner = pmem;
> - pmem->pfn_flags = PFN_DEV;
> + pmem->pfn_flags = 0;
> if (is_nd_pfn(dev)) {
> pmem->pgmap.type = MEMORY_DEVICE_FS_DAX;
> pmem->pgmap.ops = &fsdax_pagemap_ops;
> @@ -522,7 +522,6 @@ static int pmem_attach_disk(struct device *dev,
> pmem->data_offset = le64_to_cpu(pfn_sb->dataoff);
> pmem->pfn_pad = resource_size(res) -
> range_len(&pmem->pgmap.range);
> - pmem->pfn_flags |= PFN_MAP;
> bb_range = pmem->pgmap.range;
> bb_range.start += pmem->data_offset;
> } else if (pmem_should_map_pages(dev)) {
> @@ -532,7 +531,6 @@ static int pmem_attach_disk(struct device *dev,
> pmem->pgmap.type = MEMORY_DEVICE_FS_DAX;
> pmem->pgmap.ops = &fsdax_pagemap_ops;
> addr = devm_memremap_pages(dev, &pmem->pgmap);
> - pmem->pfn_flags |= PFN_MAP;
> bb_range = pmem->pgmap.range;
> } else {
> addr = devm_memremap(dev, pmem->phys_addr,
> diff --git a/fs/dax.c b/fs/dax.c
> index d35dbe1..19f444e 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -71,6 +71,11 @@ static unsigned long dax_to_pfn(void *entry)
> return xa_to_value(entry) >> DAX_SHIFT;
> }
>
> +static struct folio *dax_to_folio(void *entry)
> +{
> + return page_folio(pfn_to_page(dax_to_pfn(entry)));
> +}
> +
> static void *dax_make_entry(pfn_t pfn, unsigned long flags)
> {
> return xa_mk_value(flags | (pfn_t_to_pfn(pfn) << DAX_SHIFT));
> @@ -338,44 +343,88 @@ static unsigned long dax_entry_size(void *entry)
> return PAGE_SIZE;
> }
>
> -static unsigned long dax_end_pfn(void *entry)
> -{
> - return dax_to_pfn(entry) + dax_entry_size(entry) / PAGE_SIZE;
> -}
> -
> -/*
> - * Iterate through all mapped pfns represented by an entry, i.e. skip
> - * 'empty' and 'zero' entries.
> - */
> -#define for_each_mapped_pfn(entry, pfn) \
> - for (pfn = dax_to_pfn(entry); \
> - pfn < dax_end_pfn(entry); pfn++)
> -
> /*
> * A DAX page is considered shared if it has no mapping set and ->share (which
> * shares the ->index field) is non-zero. Note this may return false even if the
> * page is shared between multiple files but has not yet actually been mapped
> * into multiple address spaces.
> */
> -static inline bool dax_page_is_shared(struct page *page)
> +static inline bool dax_folio_is_shared(struct folio *folio)
> {
> - return !page->mapping && page->share;
> + return !folio->mapping && folio->share;
> }
>
> /*
> - * Increase the page share refcount, warning if the page is not marked as shared.
> + * Increase the folio share refcount, warning if the folio is not marked as shared.
> */
> -static inline void dax_page_share_get(struct page *page)
> +static inline void dax_folio_share_get(void *entry)
> {
> - WARN_ON_ONCE(!page->share);
> - WARN_ON_ONCE(page->mapping);
> - page->share++;
> + struct folio *folio = dax_to_folio(entry);
> +
> + WARN_ON_ONCE(!folio->share);
> + WARN_ON_ONCE(folio->mapping);
> + WARN_ON_ONCE(dax_entry_order(entry) != folio_order(folio));
> + folio->share++;
> +}
> +
> +static inline unsigned long dax_folio_share_put(struct folio *folio)
> +{
> + unsigned long ref;
> +
> + if (!dax_folio_is_shared(folio))
> + ref = 0;
> + else
> + ref = --folio->share;
> +
> + WARN_ON_ONCE(ref < 0);
> + if (!ref) {
> + folio->mapping = NULL;
> + if (folio_order(folio)) {
> + struct dev_pagemap *pgmap = page_pgmap(&folio->page);
> + unsigned int order = folio_order(folio);
> + unsigned int i;
> +
> + for (i = 0; i < (1UL << order); i++) {
> + struct page *page = folio_page(folio, i);
> +
> + ClearPageHead(page);
> + clear_compound_head(page);
> +
> + /*
> + * Reset pgmap which was over-written by
> + * prep_compound_page().
> + */
> + page_folio(page)->pgmap = pgmap;
> +
> + /* Make sure this isn't set to TAIL_MAPPING */
> + page->mapping = NULL;
> + page->share = 0;
> + WARN_ON_ONCE(page_ref_count(page));
> + }
> + }
> + }
> +
> + return ref;
> }
>
> -static inline unsigned long dax_page_share_put(struct page *page)
> +static void dax_device_folio_init(void *entry)

s/dax_device_folio_init/dax_folio_init/

...otherwise I do not see any connection to a "device" concept in this
file.

> {
> - WARN_ON_ONCE(!page->share);
> - return --page->share;
> + struct folio *folio = dax_to_folio(entry);
> + int order = dax_entry_order(entry);
> +
> + /*
> + * Folio should have been split back to order-0 pages in
> + * dax_folio_share_put() when they were removed from their
> + * final mapping.
> + */
> + WARN_ON_ONCE(folio_order(folio));
> +
> + if (order > 0) {
> + prep_compound_page(&folio->page, order);
> + if (order > 1)
> + INIT_LIST_HEAD(&folio->_deferred_list);
> + WARN_ON_ONCE(folio_ref_count(folio));
> + }
> }
>
> /*
> @@ -388,72 +437,58 @@ static inline unsigned long dax_page_share_put(struct page *page)
> * dax_holder_operations.
> */
> static void dax_associate_entry(void *entry, struct address_space *mapping,
> - struct vm_area_struct *vma, unsigned long address, bool shared)
> + struct vm_area_struct *vma, unsigned long address, bool shared)
> {
> - unsigned long size = dax_entry_size(entry), pfn, index;
> - int i = 0;
> + unsigned long size = dax_entry_size(entry), index;
> + struct folio *folio = dax_to_folio(entry);
>
> 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 && page->mapping && page->share) {
> - if (page->mapping) {
> - page->mapping = NULL;
> + if (shared && (folio->mapping || dax_folio_is_shared(folio))) {

This change in logic aligns with the previous feedback on the suspect

"if (shared && page->mapping && page->share)"

...statememt, right?

...and maybe the dax_make_shared() suggestion makes the diff smaller
here.

> + if (folio->mapping) {
> + folio->mapping = NULL;
>
> - /*
> - * Page has already been mapped into one address
> - * space so set the share count.
> - */
> - page->share = 1;
> - }
> -
> - dax_page_share_get(page);
> - } else {
> - WARN_ON_ONCE(page->mapping);
> - page->mapping = mapping;
> - page->index = index + i++;
> + /*
> + * folio has already been mapped into one address
> + * space so set the share count.
> + */
> + folio->share = 1;
> }
> +
> + dax_folio_share_get(entry);
> + } else {
> + WARN_ON_ONCE(folio->mapping);
> + dax_device_folio_init(entry);
> + folio = dax_to_folio(entry);
> + folio->mapping = mapping;
> + folio->index = index;
> }
> }
>
> static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> - bool trunc)
> + bool trunc)
> {
> - unsigned long pfn;
> + struct folio *folio = dax_to_folio(entry);
>
> if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> return;
>
> - for_each_mapped_pfn(entry, pfn) {
> - struct page *page = pfn_to_page(pfn);
> -
> - WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> - if (dax_page_is_shared(page)) {
> - /* keep the shared flag if this page is still shared */
> - if (dax_page_share_put(page) > 0)
> - continue;
> - } else
> - WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> - page->mapping = NULL;
> - page->index = 0;
> - }
> + dax_folio_share_put(folio);

Probably should not call this "share_put" anymore since it is handling
both the shared and non-shared case.

> }
>
> static struct page *dax_busy_page(void *entry)

Hmm, will this ultimately become dax_busy_folio()?

[..]
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 54b59b8..e308cb9 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -295,6 +295,8 @@ typedef struct {
> * anonymous memory.
> * @index: Offset within the file, in units of pages. For anonymous memory,
> * this is the index from the beginning of the mmap.
> + * @share: number of DAX mappings that reference this folio. See
> + * dax_associate_entry.
> * @private: Filesystem per-folio data (see folio_attach_private()).
> * @swap: Used for swp_entry_t if folio_test_swapcache().
> * @_mapcount: Do not access this member directly. Use folio_mapcount() to
> @@ -344,7 +346,10 @@ struct folio {
> struct dev_pagemap *pgmap;
> };
> struct address_space *mapping;
> - pgoff_t index;
> + union {
> + pgoff_t index;
> + unsigned long share;
> + };

This feels like it should be an immediate follow-on change if only to
separate fsdax conversion bugs from ->index ->share aliasing bugs, and
due to the significance of touching 'struct page'.

[..]

As I only have cosmetic comments you can add:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

...and here is that aformentioned patch:

-- 8< --
Subject: dcssblk: Mark DAX broken, remove FS_DAX_LIMITED support

From: Dan Williams <dan.j.williams@intel.com>

The dcssblk driver has long needed special case supoprt to enable
limited dax operation, so called CONFIG_FS_DAX_LIMITED. This mode
works around the incomplete support for ZONE_DEVICE on s390 by forgoing
the ability of dax-mapped pages to support GUP.

Now, pending cleanups to fsdax that fix its reference counting [1] depend on
the ability of all dax drivers to supply ZONE_DEVICE pages.

To allow that work to move forward, dax support needs to be paused for
dcssblk until ZONE_DEVICE support arrives. That work has been known for
a few years [2], and the removal of "pte_devmap" requirements [3] makes the
conversion easier.

For now, place the support behind CONFIG_BROKEN, and remove PFN_SPECIAL
(dcssblk was the only user).

Link: http://lore.kernel.org/cover.9f0e45d52f5cff58807831b6b867084d0b14b61c.1725941415.git-series.apopple@nvidia.com [1]
Link: http://lore.kernel.org/20210820210318.187742e8@thinkpad/ [2]
Link: http://lore.kernel.org/4511465a4f8429f45e2ac70d2e65dc5e1df1eb47.1725941415.git-series.apopple@nvidia.com [3]
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
Documentation/filesystems/dax.rst | 1 -
drivers/s390/block/Kconfig | 12 ++++++++++--
drivers/s390/block/dcssblk.c | 27 +++++++++++++++++----------
3 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst
index 719e90f1988e..08dd5e254cc5 100644
--- a/Documentation/filesystems/dax.rst
+++ b/Documentation/filesystems/dax.rst
@@ -207,7 +207,6 @@ implement direct_access.

These block devices may be used for inspiration:
- brd: RAM backed block device driver
-- dcssblk: s390 dcss block device driver
- pmem: NVDIMM persistent memory driver

diff --git a/drivers/s390/block/Kconfig b/drivers/s390/block/Kconfig
index e3710a762aba..4bfe469c04aa 100644
--- a/drivers/s390/block/Kconfig
+++ b/drivers/s390/block/Kconfig
@@ -4,13 +4,21 @@ comment "S/390 block device drivers"

config DCSSBLK
def_tristate m
- select FS_DAX_LIMITED
- select DAX
prompt "DCSSBLK support"
depends on S390 && BLOCK
help
Support for dcss block device

+config DCSSBLK_DAX
+ def_bool y
+ depends on DCSSBLK
+ # requires S390 ZONE_DEVICE support
+ depends on BROKEN
+ select DAX
+ prompt "DCSSBLK DAX support"
+ help
+ Enable DAX operation for the dcss block device
+
config DASD
def_tristate y
prompt "Support for DASD devices"
diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
index 0f14d279d30b..7248e547fefb 100644
--- a/drivers/s390/block/dcssblk.c
+++ b/drivers/s390/block/dcssblk.c
@@ -534,6 +534,21 @@ static const struct attribute_group *dcssblk_dev_attr_groups[] = {
NULL,
};

+static int dcssblk_setup_dax(struct dcssblk_dev_info *dev_info)
+{
+ struct dax_device *dax_dev;
+
+ if (!IS_ENABLED(CONFIG_DCSSBLK_DAX))
+ return 0;
+
+ dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
+ if (IS_ERR(dax_dev))
+ return PTR_ERR(dax_dev);
+ set_dax_synchronous(dax_dev);
+ dev_info->dax_dev = dax_dev;
+ return dax_add_host(dev_info->dax_dev, dev_info->gd);
+}
+
/*
* device attribute for adding devices
*/
@@ -547,7 +562,6 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
int rc, i, j, num_of_segments;
struct dcssblk_dev_info *dev_info;
struct segment_info *seg_info, *temp;
- struct dax_device *dax_dev;
char *local_buf;
unsigned long seg_byte_size;

@@ -674,14 +688,7 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
if (rc)
goto put_dev;

- dax_dev = alloc_dax(dev_info, &dcssblk_dax_ops);
- if (IS_ERR(dax_dev)) {
- rc = PTR_ERR(dax_dev);
- goto put_dev;
- }
- set_dax_synchronous(dax_dev);
- dev_info->dax_dev = dax_dev;
- rc = dax_add_host(dev_info->dax_dev, dev_info->gd);
+ rc = dcssblk_setup_dax(dev_info);
if (rc)
goto out_dax;

@@ -917,7 +924,7 @@ __dcssblk_direct_access(struct dcssblk_dev_info *dev_info, pgoff_t pgoff,
*kaddr = __va(dev_info->start + offset);
if (pfn)
*pfn = __pfn_to_pfn_t(PFN_DOWN(dev_info->start + offset),
- PFN_DEV|PFN_SPECIAL);
+ PFN_DEV);

return (dev_sz - offset) / PAGE_SIZE;
}