* Re: [PATCH v8 16/19] fsnotify: generate pre-content permission event on page fault
[not found] ` <5d0cd660-251c-423a-8828-5b836a5130f9@gmail.com>
@ 2024-12-09 10:45 ` Aithal, Srikanth
2024-12-09 12:34 ` Jan Kara
0 siblings, 1 reply; 2+ messages in thread
From: Aithal, Srikanth @ 2024-12-09 10:45 UTC (permalink / raw)
To: Klara Modin, Josef Bacik, kernel-team, linux-fsdevel, jack,
amir73il, brauner, torvalds, viro, linux-xfs, linux-btrfs,
linux-mm, linux-ext4, Linux-Next Mailing List
On 12/8/2024 10:28 PM, Klara Modin wrote:
> 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.
From linux-next20241206 onward we started hitting issues where KVM
guests running kernel > next20241206 on AMD platforms fails to shutdown,
hangs forever with below errors:
[ OK ] Reached target Late Shutdown Services.
[ OK ] Finished System Power Off.
[ OK ] Reached target System Power Off.
[ 128.946271] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Connection refused
[ 198.945362] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 298.945402] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 378.945345] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 488.945402] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 558.945904] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 632.945409] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 738.945403] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
[ 848.945342] systemd-journald[93]: Failed to send WATCHDOG=1
notification message: Transport endpoint is not connected
..
..
Bisecting the issue pointed to this patch.
commit 0790303ec869d0fd658a548551972b51ced7390c
Author: Josef Bacik <josef@toxicpanda.com>
Date: Fri Nov 15 10:30:29 2024 -0500
fsnotify: generate pre-content permission event on page fault
Same issue exists with todays linux-next build as well.
Adding below configs in the guest_config fixes the shutdown hang issue:
CONFIG_FANOTIFY=y
CONFIG_FANOTIFY_ACCESS_PERMISSIONS=y
Regards,
Srikanth Aithal
>
>> 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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