Re: [PATCH 09/12] mm: Update vm_normal_page() callers to accept FS DAX pages

Dan Williams
2024-09-27T07:18:42Z

Alistair Popple wrote:
> Currently if a PTE points to a FS DAX page vm_normal_page() will
> return NULL as these have their own special refcounting scheme. A
> future change will allow FS DAX pages to be refcounted the same as any
> other normal page.
>
> Therefore vm_normal_page() will start returning FS DAX pages. To avoid
> any change in behaviour callers that don't expect FS DAX pages will
> need to explicitly check for this. As vm_normal_page() can already
> return ZONE_DEVICE pages most callers already include a check for any
> ZONE_DEVICE page.
>
> However some callers don't, so add explicit checks where required.

I would expect justification for each of these conversions, and
hopefully with fsdax returning fully formed folios there is less need to
sprinkle these checks around.

At a minimum I think this patch needs to be broken up by file touched.

> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
> arch/x86/mm/pat/memtype.c | 4 +++-
> fs/proc/task_mmu.c | 16 ++++++++++++----
> mm/memcontrol-v1.c | 2 +-
> 3 files changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 1fa0bf6..eb84593 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -951,6 +951,7 @@ static void free_pfn_range(u64 paddr, unsigned long size)
> static int follow_phys(struct vm_area_struct *vma, unsigned long *prot,
> resource_size_t *phys)
> {
> + struct folio *folio;
> pte_t *ptep, pte;
> spinlock_t *ptl;
>
> @@ -960,7 +961,8 @@ static int follow_phys(struct vm_area_struct *vma, unsigned long *prot,
> pte = ptep_get(ptep);
>
> /* Never return PFNs of anon folios in COW mappings. */
> - if (vm_normal_folio(vma, vma->vm_start, pte)) {
> + folio = vm_normal_folio(vma, vma->vm_start, pte);
> + if (folio || (folio && !folio_is_device_dax(folio))) {

...for example, I do not immediately see why follow_phys() would need to
be careful with fsdax pages?

...but I do see why copy_page_range() (which calls follow_phys() through
track_pfn_copy()) might care. It just turns out that vma_needs_copy(),
afaics, bypasses dax MAP_SHARED mappings.

So this touch of memtype.c looks like it can be dropped.

> pte_unmap_unlock(ptep, ptl);
> return -EINVAL;
> }
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 5f171ad..456b010 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -816,6 +816,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>
> if (pte_present(ptent)) {
> page = vm_normal_page(vma, addr, ptent);
> + if (page && is_device_dax_page(page))
> + page = NULL;
> young = pte_young(ptent);
> dirty = pte_dirty(ptent);
> present = true;
> @@ -864,6 +866,8 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>
> if (pmd_present(*pmd)) {
> page = vm_normal_page_pmd(vma, addr, *pmd);
> + if (page && is_device_dax_page(page))
> + page = NULL;
> present = true;
> } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> swp_entry_t entry = pmd_to_swp_entry(*pmd);

The above can be replaced with a catch like

if (folio_test_device(folio))
return;

...in smaps_account() since ZONE_DEVICE pages are not suitable to
account as they do not reflect any memory pressure on the system memory
pool.

> @@ -1385,7 +1389,7 @@ static inline bool pte_is_pinned(struct vm_area_struct *vma, unsigned long addr,
> if (likely(!test_bit(MMF_HAS_PINNED, &vma->vm_mm->flags)))
> return false;
> folio = vm_normal_folio(vma, addr, pte);
> - if (!folio)
> + if (!folio || folio_is_device_dax(folio))
> return false;
> return folio_maybe_dma_pinned(folio);

The whole point of ZONE_DEVICE is to account for DMA so I see no reason
for pte_is_pinned() to special case dax. The caller of pte_is_pinned()
is doing it for soft_dirty reasons, and I believe soft_dirty is already
disabled for vma_is_dax(). I assume MEMORY_DEVICE_PRIVATE also does not
support soft-dirty, so I expect all ZONE_DEVICE already opt-out of this.

> }
> @@ -1710,6 +1714,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> frame = pte_pfn(pte);
> flags |= PM_PRESENT;
> page = vm_normal_page(vma, addr, pte);
> + if (page && is_device_dax_page(page))
> + page = NULL;
> if (pte_soft_dirty(pte))
> flags |= PM_SOFT_DIRTY;
> if (pte_uffd_wp(pte))
> @@ -2096,7 +2102,8 @@ static unsigned long pagemap_page_category(struct pagemap_scan_private *p,
>
> if (p->masks_of_interest & PAGE_IS_FILE) {
> page = vm_normal_page(vma, addr, pte);
> - if (page && !PageAnon(page))
> + if (page && !PageAnon(page) &&
> + !is_device_dax_page(page))
> categories |= PAGE_IS_FILE;
> }
>
> @@ -2158,7 +2165,8 @@ static unsigned long pagemap_thp_category(struct pagemap_scan_private *p,
>
> if (p->masks_of_interest & PAGE_IS_FILE) {
> page = vm_normal_page_pmd(vma, addr, pmd);
> - if (page && !PageAnon(page))
> + if (page && !PageAnon(page) &&
> + !is_device_dax_page(page))
> categories |= PAGE_IS_FILE;
> }
>
> @@ -2919,7 +2927,7 @@ static struct page *can_gather_numa_stats_pmd(pmd_t pmd,
> return NULL;
>
> page = vm_normal_page_pmd(vma, addr, pmd);
> - if (!page)
> + if (!page || is_device_dax_page(page))
> return NULL;

I am not immediately seeing a reason to block pagemap_read() from
interrogating dax-backed virtual mappings. I think these protections can
be dropped.

>
> if (PageReserved(page))
> diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
> index b37c0d8..e16053c 100644
> --- a/mm/memcontrol-v1.c
> +++ b/mm/memcontrol-v1.c
> @@ -667,7 +667,7 @@ static struct page *mc_handle_present_pte(struct vm_area_struct *vma,
> {
> struct page *page = vm_normal_page(vma, addr, ptent);
>
> - if (!page)
> + if (!page || is_device_dax_page(page))
> return NULL;
> if (PageAnon(page)) {
> if (!(mc.flags & MOVE_ANON))

I think this better handled with something like this to disable all
memcg accounting for ZONE_DEVICE pages:

diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index b37c0d870816..cfc43e8c59fe 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -940,8 +940,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
*/
if (folio_memcg(folio) == mc.from) {
ret = MC_TARGET_PAGE;
- if (folio_is_device_private(folio) ||
- folio_is_device_coherent(folio))
+ if (folio_is_device(folio))
ret = MC_TARGET_DEVICE;
if (target)
target->folio = folio;