Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic
From: David Hildenbrand
Date: Tue May 02 2023 - 13:17:34 EST
On 02.05.23 19:09, Lorenzo Stoakes wrote:
On Tue, May 02, 2023 at 05:53:46PM +0100, Lorenzo Stoakes wrote:
On Tue, May 02, 2023 at 06:38:53PM +0200, David Hildenbrand wrote:
On 02.05.23 18:34, Lorenzo Stoakes wrote:
vma_wants_writenotify() is specifically intended for setting PTE page table
flags, accounting for existing PTE flag state and whether that might
already be read-only while mixing this check with a check whether the
filesystem performs dirty tracking.
Separate out the notions of dirty tracking and a PTE write notify checking
in order that we can invoke the dirty tracking check from elsewhere.
Note that this change introduces a very small duplicate check of the
separated out vm_ops_needs_writenotify(). This is necessary to avoid making
vma_needs_dirty_tracking() needlessly complicated (e.g. passing a
check_writenotify flag or having it assume this check was already
performed). This is such a small check that it doesn't seem too egregious
to do this.
Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: Mika Penttilä <mpenttil@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
include/linux/mm.h | 1 +
mm/mmap.c | 36 +++++++++++++++++++++++++++---------
2 files changed, 28 insertions(+), 9 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 27ce77080c79..7b1d4e7393ef 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
#define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
MM_CP_UFFD_WP_RESOLVE)
+bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
{
diff --git a/mm/mmap.c b/mm/mmap.c
index 5522130ae606..295c5f2e9bd9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
}
#endif /* __ARCH_WANT_SYS_OLD_MMAP */
+/* Do VMA operations imply write notify is required? */
+static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
+{
+ return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
+}
+
+/*
+ * Does this VMA require the underlying folios to have their dirty state
+ * tracked?
+ */
+bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
+{
Sorry for not noticing this earlier, but ...
pints_owed++
Having tired eyes and jumping back and forth between tasks really seems
to start getting expensive ;)
what about MAP_PRIVATE mappings? When we write, we populate an anon page,
which will work as expected ... because we don't have to notify the fs?
I think you really also want the "If it was private or non-writable, the
write bit is already clear */" part as well and remove "false" in that case.
Not sure a 'write bit is already clear' case is relevant to checking
whether a filesystem dirty tracks? That seems specific entirely to the page
table bits.
That's why I didn't include it,
A !VM_WRITE shouldn't be GUP-writable except for FOLL_FORCE, and that
surely could be problematic if VM_MAYWRITE later?
Thinking about it though a !VM_SHARE should probably can be safely assumed
to not be dirty-trackable, so we probably do need to add a check for
!VM_SHARED -> !vma_needs_dirty_tracking
On second thoughts, we explicitly check FOLL_FORCE && !is_cow_mapping() in
check_vma_flags() so that case cannot occur.
So actually yes we should probably include this on the basis of that and
the fact that a FOLL_WRITE operation will CoW the MAP_PRIVATE mapping.
Yes, we only allow to FOLL_FORCE write to (exclusive) anonymous pages
that are mapped read-only. If it's not that, we trigger a (fake) write
fault.
--
Thanks,
David / dhildenb