On 14.07.21 18:08, Peter Xu wrote:
On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu wrote:
When a page allocated using the MAP_SHARED flag is swapped out, its pagemap
entry is cleared. In many cases, there is no difference between swapped-out
shared pages and newly allocated, non-dirty pages in the pagemap interface.
This patch addresses the behaviour and modifies pte_to_pagemap_entry() to
make use of the XArray associated with the virtual memory area struct
passed as an argument. The XArray contains the location of virtual pages
in the page cache, swap cache or on disk. If they are on either of the
caches, then the original implementation still works. If not, then the
missing information will be retrieved from the XArray.
Co-developed-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
Signed-off-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
Co-developed-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
Signed-off-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
Co-developed-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
Signed-off-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@xxxxxxxxxxx>
---
fs/proc/task_mmu.c | 37 +++++++++++++++++++++++++++++--------
1 file changed, 29 insertions(+), 8 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index eb97468dfe4c..b17c8aedd32e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1359,12 +1359,25 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
return err;
}
+static void *get_xa_entry_at_vma_addr(struct vm_area_struct *vma,
+ unsigned long addr)
+{
+ struct inode *inode = file_inode(vma->vm_file);
+ struct address_space *mapping = inode->i_mapping;
+ pgoff_t offset = linear_page_index(vma, addr);
+
+ return xa_load(&mapping->i_pages, offset);
+}
+
static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
struct vm_area_struct *vma, unsigned long addr, pte_t pte)
{
u64 frame = 0, flags = 0;
struct page *page = NULL;
+ if (vma->vm_flags & VM_SOFTDIRTY)
+ flags |= PM_SOFT_DIRTY;
+
if (pte_present(pte)) {
if (pm->show_pfn)
frame = pte_pfn(pte);
@@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
flags |= PM_SOFT_DIRTY;
if (pte_uffd_wp(pte))
flags |= PM_UFFD_WP;
- } else if (is_swap_pte(pte)) {
+ } else if (is_swap_pte(pte) || shmem_file(vma->vm_file)) {
swp_entry_t entry;
- if (pte_swp_soft_dirty(pte))
- flags |= PM_SOFT_DIRTY;
- if (pte_swp_uffd_wp(pte))
- flags |= PM_UFFD_WP;
- entry = pte_to_swp_entry(pte);
+ if (is_swap_pte(pte)) {
+ entry = pte_to_swp_entry(pte);
+ if (pte_swp_soft_dirty(pte))
+ flags |= PM_SOFT_DIRTY;
+ if (pte_swp_uffd_wp(pte))
+ flags |= PM_UFFD_WP;
+ } else {
+ void *xa_entry = get_xa_entry_at_vma_addr(vma, addr);
+
+ if (xa_is_value(xa_entry))
+ entry = radix_to_swp_entry(xa_entry);
+ else
+ goto out;
+ }
if (pm->show_pfn)
frame = swp_type(entry) |
(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
@@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
flags |= PM_FILE;
if (page && page_mapcount(page) == 1)
flags |= PM_MMAP_EXCLUSIVE;
- if (vma->vm_flags & VM_SOFTDIRTY)
- flags |= PM_SOFT_DIRTY;
IMHO moving this to the entry will only work for the initial iteration, however
it won't really help anything, as soft-dirty should always be used in pair with
clear_refs written with value "4" first otherwise all pages will be marked
soft-dirty then the pagemap data is meaningless.
After the "write 4" op VM_SOFTDIRTY will be cleared and I expect the test case
to see all zeros again even with the patch.
I think one way to fix this is to do something similar to uffd-wp: we leave a
marker in pte showing that this is soft-dirtied pte even if swapped out.
How exactly does such a pte look like? Simply pte_none() with another
bit set?
However we don't have a mechanism for that yet in current linux, and the
uffd-wp series is the first one trying to introduce something like that.
Can you give me a pointer? I'm very interested in learning how to
identify this case.
--
Thanks,
David / dhildenb
