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

[Klara Modin <klarasmodin@gmail.com>]

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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@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)

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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