The quilt patch titled
Subject: mm: allow ->huge_fault() to be called without the mmap_lock held
has been removed from the -mm tree. Its filename was
mm-allow-huge_fault-to-be-called-without-the-mmap_lock-held.patch
This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Subject: mm: allow ->huge_fault() to be called without the mmap_lock held
Date: Fri, 18 Aug 2023 21:23:34 +0100
Remove the checks for the VMA lock being held, allowing the page fault
path to call into the filesystem instead of retrying with the mmap_lock
held. This will improve scalability for DAX page faults. Also update the
documentation to match (and fix some other changes that have happened
recently).
Link: https://lkml.kernel.org/r/20230818202335.2739663-3-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/filesystems/locking.rst | 36 +++++++++++++++---------
Documentation/filesystems/porting.rst | 11 +++++++
mm/memory.c | 22 +-------------
3 files changed, 36 insertions(+), 33 deletions(-)
--- a/Documentation/filesystems/locking.rst~mm-allow-huge_fault-to-be-called-without-the-mmap_lock-held
+++ a/Documentation/filesystems/locking.rst
@@ -628,26 +628,29 @@ vm_operations_struct
prototypes::
- void (*open)(struct vm_area_struct*);
- void (*close)(struct vm_area_struct*);
- vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
+ void (*open)(struct vm_area_struct *);
+ void (*close)(struct vm_area_struct *);
+ vm_fault_t (*fault)(struct vm_fault *);
+ vm_fault_t (*huge_fault)(struct vm_fault *, unsigned int order);
+ vm_fault_t (*map_pages)(struct vm_fault *, pgoff_t start, pgoff_t end);
vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
locking rules:
-============= ========= ===========================
+============= ========== ===========================
ops mmap_lock PageLocked(page)
-============= ========= ===========================
-open: yes
-close: yes
-fault: yes can return with page locked
-map_pages: read
-page_mkwrite: yes can return with page locked
-pfn_mkwrite: yes
-access: yes
-============= ========= ===========================
+============= ========== ===========================
+open: write
+close: read/write
+fault: read can return with page locked
+huge_fault: maybe-read
+map_pages: maybe-read
+page_mkwrite: read can return with page locked
+pfn_mkwrite: read
+access: read
+============= ========== ===========================
->fault() is called when a previously not present pte is about to be faulted
in. The filesystem must find and return the page associated with the passed in
@@ -657,6 +660,13 @@ then ensure the page is not already trun
subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
locked. The VM will unlock the page.
+->huge_fault() is called when there is no PUD or PMD entry present. This
+gives the filesystem the opportunity to install a PUD or PMD sized page.
+Filesystems can also use the ->fault method to return a PMD sized page,
+so implementing this function may not be necessary. In particular,
+filesystems should not call filemap_fault() from ->huge_fault().
+The mmap_lock may not be held when this method is called.
+
->map_pages() is called when VM asks to map easy accessible pages.
Filesystem should find and map pages associated with offsets from "start_pgoff"
till "end_pgoff". ->map_pages() is called with the RCU lock held and must
--- a/Documentation/filesystems/porting.rst~mm-allow-huge_fault-to-be-called-without-the-mmap_lock-held
+++ a/Documentation/filesystems/porting.rst
@@ -943,3 +943,14 @@ file pointer instead of struct dentry po
changed to simplify callers. The passed file is in a non-open state and on
success must be opened before returning (e.g. by calling
finish_open_simple()).
+
+---
+
+**mandatory**
+
+Calling convention for ->huge_fault has changed. It now takes a page
+order instead of an enum page_entry_size, and it may be called without the
+mmap_lock held. All in-tree users have been audited and do not seem to
+depend on the mmap_lock being held, but out of tree users should verify
+for themselves. If they do need it, they can return VM_FAULT_RETRY to
+be called with the mmap_lock held.
--- a/mm/memory.c~mm-allow-huge_fault-to-be-called-without-the-mmap_lock-held
+++ a/mm/memory.c
@@ -4854,13 +4854,8 @@ static inline vm_fault_t create_huge_pmd
struct vm_area_struct *vma = vmf->vma;
if (vma_is_anonymous(vma))
return do_huge_pmd_anonymous_page(vmf);
- if (vma->vm_ops->huge_fault) {
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
- vma_end_read(vma);
- return VM_FAULT_RETRY;
- }
+ if (vma->vm_ops->huge_fault)
return vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
- }
return VM_FAULT_FALLBACK;
}
@@ -4880,10 +4875,6 @@ static inline vm_fault_t wp_huge_pmd(str
if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
if (vma->vm_ops->huge_fault) {
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
- vma_end_read(vma);
- return VM_FAULT_RETRY;
- }
ret = vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
if (!(ret & VM_FAULT_FALLBACK))
return ret;
@@ -4904,13 +4895,8 @@ static vm_fault_t create_huge_pud(struct
/* No support for anonymous transparent PUD pages yet */
if (vma_is_anonymous(vma))
return VM_FAULT_FALLBACK;
- if (vma->vm_ops->huge_fault) {
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
- vma_end_read(vma);
- return VM_FAULT_RETRY;
- }
+ if (vma->vm_ops->huge_fault)
return vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
- }
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
return VM_FAULT_FALLBACK;
}
@@ -4927,10 +4913,6 @@ static vm_fault_t wp_huge_pud(struct vm_
goto split;
if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
if (vma->vm_ops->huge_fault) {
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
- vma_end_read(vma);
- return VM_FAULT_RETRY;
- }
ret = vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
if (!(ret & VM_FAULT_FALLBACK))
return ret;
_
Patches currently in -mm which might be from willy@xxxxxxxxxxxxx are
