[Linux-cachefs] [PATCH v13 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

[Yafang Shao]

On Fri, Dec 18, 2020 at 8:14 AM Dave Chinner <david at fromorbit.com> wrote:
>
> On Thu, Dec 17, 2020 at 09:11:57AM +0800, Yafang Shao wrote:
> > PF_FSTRANS which is used to avoid transaction reservation recursion, is
> > dropped since commit 9070733b4efa ("xfs: abstract PF_FSTRANS to
> > PF_MEMALLOC_NOFS") and replaced by PF_MEMALLOC_NOFS which means to avoid
> > filesystem reclaim recursion.
> >
> > As these two flags have different meanings, we'd better reintroduce
> > PF_FSTRANS back. To avoid wasting the space of PF_* flags in task_struct,
> > we can reuse the current->journal_info to do that, per Willy. As the
> > check of transaction reservation recursion is used by XFS only, we can
> > move the check into xfs_vm_writepage(s), per Dave.
> >
> > Cc: Darrick J. Wong <darrick.wong at oracle.com>
> > Cc: Matthew Wilcox (Oracle) <willy at infradead.org>
> > Cc: Christoph Hellwig <hch at lst.de>
> > Cc: Dave Chinner <david at fromorbit.com>
> > Cc: Michal Hocko <mhocko at kernel.org>
> > Cc: David Howells <dhowells at redhat.com>
> > Cc: Jeff Layton <jlayton at redhat.com>
> > Signed-off-by: Yafang Shao <laoar.shao at gmail.com>
> > ---
> >  fs/iomap/buffered-io.c |  7 -------
> >  fs/xfs/xfs_aops.c      | 17 +++++++++++++++++
> >  fs/xfs/xfs_trans.h     | 26 +++++++++++++++++++-------
> >  3 files changed, 36 insertions(+), 14 deletions(-)
> >
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 10cc7979ce38..3c53fa6ce64d 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -1458,13 +1458,6 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
> >                       PF_MEMALLOC))
> >               goto redirty;
> >
> > -     /*
> > -      * Given that we do not allow direct reclaim to call us, we should
> > -      * never be called in a recursive filesystem reclaim context.
> > -      */
> > -     if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> > -             goto redirty;
> > -
> >       /*
> >        * Is this page beyond the end of the file?
> >        *
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 2371187b7615..0da0242d42c3 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -568,6 +568,16 @@ xfs_vm_writepage(
> >  {
> >       struct xfs_writepage_ctx wpc = { };
> >
> > +     /*
> > +      * Given that we do not allow direct reclaim to call us, we should
> > +      * never be called while in a filesystem transaction.
> > +      */
>
> Comment is wrong. This is not protecting against direct reclaim
> recursion, this is protecting against writeback from within a
> transaction context.
>

Ah, I forgot to change this comment after copy and paste. Thanks for
pointing it out.

> Best to remove the comment altogether, because it is largely
> redundant.
>

Sure, I will remove these comments.

> > +     if (WARN_ON_ONCE(xfs_trans_context_active())) {
> > +             redirty_page_for_writepage(wbc, page);
> > +             unlock_page(page);
> > +             return 0;
> > +     }
> > +
> >       return iomap_writepage(page, wbc, &wpc.ctx, &xfs_writeback_ops);
> >  }
> >
> > @@ -579,6 +589,13 @@ xfs_vm_writepages(
> >       struct xfs_writepage_ctx wpc = { };
> >
> >       xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
> > +     /*
> > +      * Given that we do not allow direct reclaim to call us, we should
> > +      * never be called while in a filesystem transaction.
> > +      */
>
> same here.
>
> > +     if (WARN_ON_ONCE(xfs_trans_context_active()))
> > +             return 0;
> > +
> >       return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
> >  }
> >
> > diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> > index 12380eaaf7ce..0c8140147b9b 100644
> > --- a/fs/xfs/xfs_trans.h
> > +++ b/fs/xfs/xfs_trans.h
> > @@ -268,29 +268,41 @@ xfs_trans_item_relog(
> >       return lip->li_ops->iop_relog(lip, tp);
> >  }
> >
> > +static inline bool
> > +xfs_trans_context_active(void)
> > +{
> > +     /* Use journal_info to indicate current is in a transaction */
> > +     return current->journal_info != NULL;
> > +}
>
> Comment is not necessary.
>
> > +
> >  static inline void
> >  xfs_trans_context_set(struct xfs_trans *tp)
> >  {
> > +     ASSERT(!current->journal_info);
> > +     current->journal_info = tp;
> >       tp->t_pflags = memalloc_nofs_save();
> >  }
> >
> >  static inline void
> >  xfs_trans_context_clear(struct xfs_trans *tp)
> >  {
> > +     /*
> > +      * If xfs_trans_context_swap() handed the NOFS context to a
> > +      * new transaction we do not clear the context here.
> > +      */
>
> It's a transaction context, not a "NOFS context". Setting NOFS is
> just something we implement inside the transaction context. More
> correct would be:
>
>         /*
>          * If we handed over the context via xfs_trans_context_swap() then
>          * the context is no longer ours to clear.
>          */
>

Sure, I will change it.


-- 
Thanks
Yafang