[PATCH v8 16/19] fsnotify: generate pre-content permission event on page fault

Josef Bacik
2024-11-15T15:31:55Z

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.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
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)
+{
+ 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);
+ 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
--
2.43.0