Re: [PATCH v13 1/4] mm: Add become_kswapd and restore_kswapd

Yafang Shao
2020-12-17T04:47:26Z

On Thu, Dec 17, 2020 at 11:06 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Thu, Dec 17, 2020 at 09:11:54AM +0800, Yafang Shao wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >
> > Since XFS needs to pretend to be kswapd in some of its worker threads,
> > create methods to save & restore kswapd state. Don't bother restoring
> > kswapd state in kswapd -- the only time we reach this code is when we're
> > exiting and the task_struct is about to be destroyed anyway.
> >
> > Cc: Dave Chinner <david@fromorbit.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> > fs/xfs/libxfs/xfs_btree.c | 14 ++++++++------
> > include/linux/sched/mm.h | 23 +++++++++++++++++++++++
> > mm/vmscan.c | 16 +---------------
> > 3 files changed, 32 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> > index 51dbff9b0908..0f35b7a38e76 100644
> > --- a/fs/xfs/libxfs/xfs_btree.c
> > +++ b/fs/xfs/libxfs/xfs_btree.c
> > @@ -2813,8 +2813,9 @@ xfs_btree_split_worker(
> > {
> > struct xfs_btree_split_args *args = container_of(work,
> > struct xfs_btree_split_args, work);
> > + bool is_kswapd = args->kswapd;
> > unsigned long pflags;
> > - unsigned long new_pflags = PF_MEMALLOC_NOFS;
> > + int memalloc_nofs;
> >
> > /*
> > * we are in a transaction context here, but may also be doing work
> > @@ -2822,16 +2823,17 @@ xfs_btree_split_worker(
> > * temporarily to ensure that we don't block waiting for memory reclaim
> > * in any way.
> > */
> > - if (args->kswapd)
> > - new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> > -
> > - current_set_flags_nested(&pflags, new_pflags);
> > + if (is_kswapd)
> > + pflags = become_kswapd();
> > + memalloc_nofs = memalloc_nofs_save();
> >
> > args->result = __xfs_btree_split(args->cur, args->level, args->ptrp,
> > args->key, args->curp, args->stat);
> > complete(args->done);
> >
> > - current_restore_flags_nested(&pflags, new_pflags);
> > + memalloc_nofs_restore(memalloc_nofs);
> > + if (is_kswapd)
> > + restore_kswapd(pflags);
> > }
> >
> > /*
> > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> > index d5ece7a9a403..2faf03e79a1e 100644
> > --- a/include/linux/sched/mm.h
> > +++ b/include/linux/sched/mm.h
> > @@ -278,6 +278,29 @@ static inline void memalloc_nocma_restore(unsigned int flags)
> > }
> > #endif
> >
> > +/*
> > + * Tell the memory management code that this thread is working on behalf
> > + * of background memory reclaim (like kswapd). That means that it will
> > + * get access to memory reserves should it need to allocate memory in
> > + * order to make forward progress. With this great power comes great
> > + * responsibility to not exhaust those reserves.
> > + */
> > +#define KSWAPD_PF_FLAGS (PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD)
> > +
> > +static inline unsigned long become_kswapd(void)
> > +{
> > + unsigned long flags = current->flags & KSWAPD_PF_FLAGS;
> > +
> > + current->flags |= KSWAPD_PF_FLAGS;
> > +
> > + return flags;
> > +}
>
> You can get rid of the empty lines out of this function.
>
> > +static inline void restore_kswapd(unsigned long flags)
> > +{
> > + current->flags &= ~(flags ^ KSWAPD_PF_FLAGS);
> > +}
>
> Urk, that requires thinking about to determine whether it is
> correct. And it is 3 runtime logic operations (^, ~ and &) too. The
> way all the memalloc_*_restore() functions restore the previous
> flags is obviously correct and only requires 2 runtime logic
> operations because the compiler calculates the ~ operation on the
> constant. So why do it differently here? i.e.:
>
> current->flags = (current->flags & ~KSWAPD_PF_FLAGS) | flags;
>

I will change it as you suggested if Matthew doesn't have a different
opinion, Matthew ?

> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3870,19 +3870,7 @@ static int kswapd(void *p)
> > if (!cpumask_empty(cpumask))
> > set_cpus_allowed_ptr(tsk, cpumask);
> >
> > - /*
> > - * Tell the memory management that we're a "memory allocator",
> > - * and that if we need more memory we should get access to it
> > - * regardless (see "__alloc_pages()"). "kswapd" should
> > - * never get caught in the normal page freeing logic.
> > - *
> > - * (Kswapd normally doesn't need memory anyway, but sometimes
> > - * you need a small amount of memory in order to be able to
> > - * page out something else, and this flag essentially protects
> > - * us from recursively trying to free more memory as we're
> > - * trying to free the first piece of memory in the first place).
> > - */
> > - tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> > + become_kswapd();
> > set_freezable();
> >
> > WRITE_ONCE(pgdat->kswapd_order, 0);
> > @@ -3932,8 +3920,6 @@ static int kswapd(void *p)
> > goto kswapd_try_sleep;
> > }
> >
> > - tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
> > -
>
> Missing a restore_kswapd()?
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com

--
Thanks
Yafang