Hi, On 2024-11-15 16:30, Josef Bacik wrote:
FS_PRE_ACCESS or FS_PRE_MODIFY will be generated on page fault depending on the faulting method. This pre-content event is meant to be used by hierarchical storage managers that want to fill in the file content on first read access. Export a simple helper that file systems that have their own ->fault() will use, and have a more complicated helper to be do fancy things with in filemap_fault.
This patch (0790303ec869d0fd658a548551972b51ced7390c in next-20241206) interacts poorly with some programs which hang and are stuck at 100 % sys cpu usage (examples of programs are logrotate and atop with root privileges).
I also retested the new version on Jan Kara's for_next branch and it behaves the same way.
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx> --- include/linux/mm.h | 1 + mm/filemap.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 79 insertions(+) diff --git a/include/linux/mm.h b/include/linux/mm.h index 01c5e7a4489f..90155ef8599a 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3406,6 +3406,7 @@ extern vm_fault_t filemap_fault(struct vm_fault *vmf); extern vm_fault_t filemap_map_pages(struct vm_fault *vmf, pgoff_t start_pgoff, pgoff_t end_pgoff); extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf); +extern vm_fault_t filemap_fsnotify_fault(struct vm_fault *vmf);extern unsigned long stack_guard_gap;/* Generic expand stack which grows the stack according to GROWS{UP,DOWN} */ diff --git a/mm/filemap.c b/mm/filemap.c index 68ea596f6905..0bf7d645dec5 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -47,6 +47,7 @@ #include <linux/splice.h> #include <linux/rcupdate_wait.h> #include <linux/sched/mm.h> +#include <linux/fsnotify.h> #include <asm/pgalloc.h> #include <asm/tlbflush.h> #include "internal.h" @@ -3289,6 +3290,52 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf) return ret; }+/**+ * filemap_fsnotify_fault - maybe emit a pre-content event. + * @vmf: struct vm_fault containing details of the fault. + * @folio: the folio we're faulting in. + * + * If we have a pre-content watch on this file we will emit an event for this + * range. If we return anything the fault caller should return immediately, we + * will return VM_FAULT_RETRY if we had to emit an event, which will trigger the + * fault again and then the fault handler will run the second time through. + * + * This is meant to be called with the folio that we will be filling in to make + * sure the event is emitted for the correct range. + * + * Return: a bitwise-OR of %VM_FAULT_ codes, 0 if nothing happened. + */ +vm_fault_t filemap_fsnotify_fault(struct vm_fault *vmf)
The parameters mentioned above do not seem to match with the function.
+{
+ struct file *fpin = NULL;
+ int mask = (vmf->flags & FAULT_FLAG_WRITE) ? MAY_WRITE : MAY_ACCESS;
+ loff_t pos = vmf->pgoff >> PAGE_SHIFT;
+ size_t count = PAGE_SIZE;
+ vm_fault_t ret;
+
+ /*
+ * We already did this and now we're retrying with everything locked,
+ * don't emit the event and continue.
+ */
+ if (vmf->flags & FAULT_FLAG_TRIED)
+ return 0;
+
+ /* No watches, we're done. */
+ if (!fsnotify_file_has_pre_content_watches(vmf->vma->vm_file))
+ return 0;
+
+ fpin = maybe_unlock_mmap_for_io(vmf, fpin);
+ if (!fpin)
+ return VM_FAULT_SIGBUS;
+
+ ret = fsnotify_file_area_perm(fpin, mask, &pos, count);
+ fput(fpin);
+ if (ret)
+ return VM_FAULT_SIGBUS;
+ return VM_FAULT_RETRY;
+}
+EXPORT_SYMBOL_GPL(filemap_fsnotify_fault);
+
/**
* filemap_fault - read in file data for page fault handling
* @vmf: struct vm_fault containing details of the fault
@@ -3392,6 +3439,37 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
* or because readahead was otherwise unable to retrieve it.
*/
if (unlikely(!folio_test_uptodate(folio))) {
+ /*
+ * If this is a precontent file we have can now emit an event to
+ * try and populate the folio.
+ */
+ if (!(vmf->flags & FAULT_FLAG_TRIED) &&
+ fsnotify_file_has_pre_content_watches(file)) {
+ loff_t pos = folio_pos(folio);
+ size_t count = folio_size(folio);
+
+ /* We're NOWAIT, we have to retry. */
+ if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT) {
+ folio_unlock(folio);
+ goto out_retry;
+ }
+
+ if (mapping_locked)
+ filemap_invalidate_unlock_shared(mapping);
+ mapping_locked = false;
+
+ folio_unlock(folio);
+ fpin = maybe_unlock_mmap_for_io(vmf, fpin);
When I look at it with GDB it seems to get here, but then always jumps to out_retry, which keeps happening when it reenters, and never seems to progress beyond from what I could tell.
For logrotate, strace stops at "mmap(NULL, 909, PROT_READ, MAP_PRIVATE|MAP_POPULATE, 3, 0".
For atop, strace stops at "mlockall(MCL_CURRENT|MCL_FUTURE". If I remove this entire patch snippet everything seems to be normal.
+ if (!fpin) + goto out_retry; + + error = fsnotify_file_area_perm(fpin, MAY_ACCESS, &pos, + count); + if (error) + ret = VM_FAULT_SIGBUS; + goto out_retry; + } + /* * If the invalidate lock is not held, the folio was in cache * and uptodate and now it is not. Strange but possible since we
Please let me know if there's anything else you need. Regards, Klara Modin
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# bad: [084d3ed79399b308a21bcd0a7f009db6bd57ff38] ext4: enable large folio for regular file git bisect start 'HEAD' # status: waiting for good commit(s), bad commit known # good: [b8f52214c61a5b99a54168145378e91b40d10c90] Merge tag 'audit-pr-20241205' of git://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/audit git bisect good b8f52214c61a5b99a54168145378e91b40d10c90 # bad: [0413a93cd7f4ed0d625f38094746b342b9456e8c] Merge branch 'hwmon-next' of git://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging.git git bisect bad 0413a93cd7f4ed0d625f38094746b342b9456e8c # good: [8e073f624bcaada15339e92bb8b22523324f403f] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/joel/bmc.git git bisect good 8e073f624bcaada15339e92bb8b22523324f403f # bad: [2c230f2d1a4cf25414d445c8983bf9ff90fc3a2d] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/rw/ubifs.git git bisect bad 2c230f2d1a4cf25414d445c8983bf9ff90fc3a2d # good: [9a4bc7289685088172122d5579886db94c384510] bcachefs: Go RW earlier, for normal rw mount git bisect good 9a4bc7289685088172122d5579886db94c384510 # good: [4fdad3e3cc21679d4d61bb0b796fc86b7568c08e] Merge branch 'for-next-next-v6.13-20241203' into for-next-20241203 git bisect good 4fdad3e3cc21679d4d61bb0b796fc86b7568c08e # good: [fb92391ef70a46f9f17b766115075dc8c7cef44c] bcachefs: Call bch2_btree_lost_data() on btree read error git bisect good fb92391ef70a46f9f17b766115075dc8c7cef44c # bad: [78765a8ed5f98d66d5725b4ecfa32114d88f89fc] Merge fanotify HSM implementation. git bisect bad 78765a8ed5f98d66d5725b4ecfa32114d88f89fc # good: [6cc9cb93ac86ada0af3c5f132075b5b6ca7e85dd] fanotify: report file range info with pre-content events git bisect good 6cc9cb93ac86ada0af3c5f132075b5b6ca7e85dd # bad: [d8d33c43e1fb472890f7d97d0026d478d2b6fa36] xfs: add pre-content fsnotify hook for DAX faults git bisect bad d8d33c43e1fb472890f7d97d0026d478d2b6fa36 # good: [9dee1a117266990083bf514038a19017bdc21497] fanotify: disable readahead if we have pre-content watches git bisect good 9dee1a117266990083bf514038a19017bdc21497 # bad: [0790303ec869d0fd658a548551972b51ced7390c] fsnotify: generate pre-content permission event on page fault git bisect bad 0790303ec869d0fd658a548551972b51ced7390c # good: [6199ec7fa1ddb7e9c546ae8e549da904a3afb688] mm: don't allow huge faults for files with pre content watches git bisect good 6199ec7fa1ddb7e9c546ae8e549da904a3afb688 # first bad commit: [0790303ec869d0fd658a548551972b51ced7390c] fsnotify: generate pre-content permission event on page fault
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