[PATCH v3] Fix ext4 fault handling when mounted with -o dax,ro

[rdodgen]

From: Randy Dodgen 

If an ext4 filesystem is mounted with both the DAX and read-only
options, executables on that filesystem will fail to start (claiming
'Segmentation fault') due to the fault handler returning
VM_FAULT_SIGBUS.

This is due to the DAX fault handler (see ext4_dax_huge_fault)
attempting to write to the journal when FAULT_FLAG_WRITE is set. This is
the wrong behavior for write faults which will lead to a COW page; in
particular, this fails for readonly mounts.

This change avoids journal writes for faults that are expected to COW.

It might be the case that this could be better handled in
ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside
dax_iomap_fault). These is some overlap already (e.g. grabbing journal
handles).

Signed-off-by: Randy Dodgen 
---

This version is simplified as suggested by Ross; all fault sizes and fallbacks
are handled by dax_iomap_fault.

 fs/ext4/file.c | 15 ++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0d7cf0cc9b87..dc1e1fb6b54c 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -279,7 +279,20 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf,
handle_t *handle = NULL;
struct inode *inode = file_inode(vmf->vma->vm_file);
struct super_block *sb = inode->i_sb;
-   bool write = vmf->flags & FAULT_FLAG_WRITE;
+
+   /*
+* We have to distinguish real writes from writes which will result in a
+* COW page; COW writes should *not* poke the journal (the file will not
+* be changed). Doing so would cause unintended failures when mounted
+* read-only.
+*
+* We check for VM_SHARED rather than vmf->cow_page since the latter is
+* unset for pe_size != PE_SIZE_PTE (i.e. only in do_cow_fault); for
+* other sizes, dax_iomap_fault will handle splitting / fallback so that
+* we eventually come back with a COW page.
+*/
+   bool write = (vmf->flags & FAULT_FLAG_WRITE) &&
+   (vmf->vma->vm_flags & VM_SHARED);
 
if (write) {
sb_start_pagefault(sb);
-- 
2.14.1.342.g6490525c54-goog

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[PATCH v2] Fix ext4 fault handling when mounted with -o dax,ro

[rdodgen]

From: Randy Dodgen 

If an ext4 filesystem is mounted with both the DAX and read-only
options, executables on that filesystem will fail to start (claiming
'Segmentation fault') due to the fault handler returning
VM_FAULT_SIGBUS.

This is due to the DAX fault handler (see ext4_dax_huge_fault)
attempting to write to the journal when FAULT_FLAG_WRITE is set. This is
the wrong behavior for write faults which will lead to a COW page; in
particular, this fails for readonly mounts.

This changes replicates some check from dax_iomap_fault to more
precisely reason about when a journal-write is needed.

It might be the case that this could be better handled in
ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside
dax_iomap_fault). These is some overlap already (e.g. grabbing journal
handles).

Signed-off-by: Randy Dodgen 
---

I'm resending for some DMARC-proofing (thanks Ted for the explanation), a
missing Signed-off-by, and some extra cc's. Oops!

 fs/ext4/file.c | 26 +-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0d7cf0cc9b87..d512fb85a3e3 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf,
handle_t *handle = NULL;
struct inode *inode = file_inode(vmf->vma->vm_file);
struct super_block *sb = inode->i_sb;
-   bool write = vmf->flags & FAULT_FLAG_WRITE;
+   bool write;
+
+   /*
+* We have to distinguish real writes from writes which will result in a
+* COW page
+* - COW writes need to fall-back to installing PTEs. See
+*   dax_iomap_pmd_fault.
+* - COW writes should *not* poke the journal (the file will not be
+*   changed). Doing so would cause unintended failures when mounted
+*   read-only.
+*/
+   if (pe_size == PE_SIZE_PTE) {
+   /* See dax_iomap_pte_fault. */
+   write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page;
+   } else if (pe_size == PE_SIZE_PMD) {
+   /* See dax_iomap_pmd_fault. */
+   write = vmf->flags & FAULT_FLAG_WRITE;
+   if (write && !(vmf->vma->vm_flags & VM_SHARED)) {
+   split_huge_pmd(vmf->vma, vmf->pmd, vmf->address);
+   count_vm_event(THP_FAULT_FALLBACK);
+   return VM_FAULT_FALLBACK;
+   }
+   } else {
+   return VM_FAULT_FALLBACK;
+   }
 
if (write) {
sb_start_pagefault(sb);
-- 
2.14.1.480.gb18f417b89-goog

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