Re: [PATCH v2 13/16] fsnotify: generate pre-content permission event on page fault

Amir Goldstein
2024-08-09T10:34:47Z

On Thu, Aug 8, 2024 at 9:28 PM Josef Bacik <josef@toxicpanda.com> 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.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> include/linux/mm.h | 2 +
> mm/filemap.c | 97 ++++++++++++++++++++++++++++++++++++++++++----
> 2 files changed, 92 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ab3d78116043..c33f3b7f7261 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3503,6 +3503,8 @@ 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_maybe_emit_fsnotify_event(struct vm_fault *vmf,
> + struct file **fpin);
>
> 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 8b1684b62177..3d232166b051 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -46,6 +46,7 @@
> #include <linux/pipe_fs_i.h>
> #include <linux/splice.h>
> #include <linux/rcupdate_wait.h>
> +#include <linux/fsnotify.h>
> #include <asm/pgalloc.h>
> #include <asm/tlbflush.h>
> #include "internal.h"
> @@ -3112,13 +3113,13 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
> * that. If we didn't pin a file then we return NULL. The file that is
> * returned needs to be fput()'ed when we're done with it.
> */
> -static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> +static struct file *do_sync_mmap_readahead(struct vm_fault *vmf,
> + struct file *fpin)
> {
> struct file *file = vmf->vma->vm_file;
> struct file_ra_state *ra = &file->f_ra;
> struct address_space *mapping = file->f_mapping;
> DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
> - struct file *fpin = NULL;
> unsigned long vm_flags = vmf->vma->vm_flags;
> unsigned int mmap_miss;
>
> @@ -3190,12 +3191,12 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> * was pinned if we have to drop the mmap_lock in order to do IO.
> */
> static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
> - struct folio *folio)
> + struct folio *folio,
> + struct file *fpin)
> {
> struct file *file = vmf->vma->vm_file;
> struct file_ra_state *ra = &file->f_ra;
> DEFINE_READAHEAD(ractl, file, ra, file->f_mapping, vmf->pgoff);
> - struct file *fpin = NULL;
> unsigned int mmap_miss;
>
> /* See comment in do_sync_mmap_readahead. */
> @@ -3260,6 +3261,72 @@ static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
> return ret;
> }
>
> +/**
> + * filemap_maybe_emit_fsnotify_event - maybe emit a pre-content event.
> + * @vmf: struct vm_fault containing details of the fault.
> + * @fpin: pointer to the struct file pointer that may be pinned.
> + *
> + * If we have pre-content watches on this file we will need to emit an event for
> + * this range. We will handle dropping the lock and emitting the event.
> + *
> + * If FAULT_FLAG_RETRY_NOWAIT is set then we'll return VM_FAULT_RETRY.
> + *
> + * If no event was emitted then *fpin will be NULL and we will return 0.
> + *
> + * If any error occurred we will return VM_FAULT_SIGBUS, *fpin could still be
> + * set and will need to have fput() called on it.
> + *
> + * If we emitted the event then we will return 0 and *fpin will be set, this
> + * must have fput() called on it, and the caller must call VM_FAULT_RETRY after
> + * any other operations it does in order to re-fault the page and make sure the
> + * appropriate locking is maintained.
> + *
> + * Return: the appropriate vm_fault_t return code, 0 on success.
> + */
> +vm_fault_t filemap_maybe_emit_fsnotify_event(struct vm_fault *vmf,
> + struct file **fpin)
> +{
> + struct file *file = vmf->vma->vm_file;
> + loff_t pos = vmf->pgoff << PAGE_SHIFT;
> + int mask = (vmf->flags & FAULT_FLAG_WRITE) ? MAY_WRITE : MAY_READ;

You missed my comment about using MAY_ACCESS here
and alter fsnotify hook, so legacy FAN_ACCESS_PERM event
won't be generated from page fault.

Thanks,
Amir.

> + int 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, return NULL. */
> + if (!fsnotify_file_has_pre_content_watches(file))
> + return 0;
> +
> + /* We are NOWAIT, we can't wait, just return EAGAIN. */
> + if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
> + return VM_FAULT_RETRY;
> +
> + /*
> + * If this fails then we're not allowed to drop the fault lock, return a
> + * SIGBUS so we don't errantly populate pagecache with bogus data for
> + * this file.
> + */
> + *fpin = maybe_unlock_mmap_for_io(vmf, *fpin);
> + if (*fpin == NULL)
> + return VM_FAULT_SIGBUS | VM_FAULT_RETRY;
> +
> + /*
> + * We can't fput(*fpin) at this point because we could have been passed
> + * in fpin from a previous call.
> + */
> + ret = fsnotify_file_area_perm(*fpin, mask, &pos, PAGE_SIZE);
> + if (ret)
> + return VM_FAULT_SIGBUS;
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(filemap_maybe_emit_fsnotify_event);
> +
> /**
> * filemap_fault - read in file data for page fault handling
> * @vmf: struct vm_fault containing details of the fault
> @@ -3299,6 +3366,19 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> if (unlikely(index >= max_idx))
> return VM_FAULT_SIGBUS;
>
> + /*
> + * If we have pre-content watchers then we need to generate events on
> + * page fault so that we can populate any data before the fault.
> + */
> + ret = filemap_maybe_emit_fsnotify_event(vmf, &fpin);
> + if (unlikely(ret)) {
> + if (fpin) {
> + fput(fpin);
> + ret |= VM_FAULT_RETRY;
> + }
> + return ret;
> + }
> +
> /*
> * Do we have something in the page cache already?
> */
> @@ -3309,21 +3389,24 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> * the lock.
> */
> if (!(vmf->flags & FAULT_FLAG_TRIED))
> - fpin = do_async_mmap_readahead(vmf, folio);
> + fpin = do_async_mmap_readahead(vmf, folio, fpin);
> if (unlikely(!folio_test_uptodate(folio))) {
> filemap_invalidate_lock_shared(mapping);
> mapping_locked = true;
> }
> } else {
> ret = filemap_fault_recheck_pte_none(vmf);
> - if (unlikely(ret))
> + if (unlikely(ret)) {
> + if (fpin)
> + goto out_retry;
> return ret;
> + }
>
> /* No page in the page cache at all */
> count_vm_event(PGMAJFAULT);
> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
> ret = VM_FAULT_MAJOR;
> - fpin = do_sync_mmap_readahead(vmf);
> + fpin = do_sync_mmap_readahead(vmf, fpin);
> retry_find:
> /*
> * See comment in filemap_create_folio() why we need
> --
> 2.43.0
>