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 8mT2Eh9SkH+YVOmaKsajgZgrBxA7fWmGoxXswEVxJIFj3vW7yNc0C5HaUdYa5iGOMs4kg2ht4s7yy7NRQuh7BifWjo6BQ6k4S1H+6axZucxhSV1L6zN9d+lr3Xo/vy1unzA== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.3 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-04-01 at 13:20 +0200, Jan Kara wrote: > On Mon 31-03-25 21:13:20, James Bottomley wrote: > > On Tue, 2025-04-01 at 01:32 +0200, Christian Brauner wrote: [...] > > > diff --git a/include/linux/fs.h b/include/linux/fs.h > > > index b379a46b5576..528e73f192ac 100644 > > > --- a/include/linux/fs.h > > > +++ b/include/linux/fs.h > > > @@ -1782,7 +1782,8 @@ static inline void __sb_end_write(struct > > > super_block *sb, int level) > > > =C2=A0static inline void __sb_start_write(struct super_block *sb, int > > > level) > > > =C2=A0{ > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 percpu_down_read_freezable= (sb->s_writers.rw_sem + level - > > > 1, > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 level =3D=3D SB_F= REEZE_WRITE); > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (level =3D=3D SB_= FREEZE_WRITE || > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 level =3D= =3D > > > SB_FREEZE_PAGEFAULT)); > > > =C2=A0} > >=20 > > Yes, I was about to tell Jan that the condition here simply needs > > to be true.=C2=A0 All our rwsem levels need to be freezable to avoid a > > hibernation failure. >=20 > So there is one snag with this. SB_FREEZE_PAGEFAULT level is acquired > under mmap_sem, SB_FREEZE_INTERNAL level is possibly acquired under > some other filesystem locks. Just for SB_FREEZE_INTERNAL, I think there's no case of sb_start_intwrite() that can ever hold in D wait because by the time we acquire the semaphore for write, the internal freeze_fs should have been called and the filesystem should have quiesced itself. On the other hand, if that theory itself is true, there's no real need for sb_start_intwrite() at all because it can never conflict. > So if you freeze the filesystem, a task can block on frozen > filesystem with e.g. mmap_sem held and if some other task then blocks > on grabbing that mmap_sem, hibernation fails because we'll be unable > to hibernate the task waiting for mmap_sem. So if you'd like to > completely avoid these hibernation failures, you'd have to make a > slew of filesystem related locks use freezable sleeping. I don't > think that's feasible. I wouldn't see that because I'm on x86_64 and that takes the vma_lock in page faults not the mmap_lock. The granularity of all these locks is process level, so it's hard to see what they'd be racing with ... even if I conjecture two threads trying to write to something, they'd have to have some internal co-ordination which would likely prevent the second one from writing if the first got stuck on the page fault.=20 > I was hoping that failures due to SB_FREEZE_PAGEFAULT level not being > freezable would be rare enough but you've proven they are quite > frequent. We can try making SB_FREEZE_PAGEFAULT level (or even > SB_FREEZE_INTERNAL) freezable and see whether that works good > enough... I'll try to construct a more severe test than systemd-journald ... it looks to be single threaded in its operation. Regards, James