Re: [PATCH v11 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Dec 9, 2020 at 3:00 AM Darrick J. Wong <darrick.wong@oracle.com> wrote:
>
> On Tue, Dec 08, 2020 at 08:28:24PM +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 commit 7dea19f9ee63 ("mm: introduce
> > memalloc_nofs_{save,restore} API") 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.
> >
> > To better abstract that behavoir, two new helpers are introduced, as
> > follows,
> > - xfs_trans_context_active
> >   To check whehter current is in fs transcation or not
> > - xfs_trans_context_swap
> >   Transfer the transaction context when rolling a permanent transaction
> >
> > These two new helpers are instroduced in xfs_trans.h.
> >
> > Cc: Darrick J. Wong <darrick.wong@oracle.com>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Dave Chinner <david@fromorbit.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Jeff Layton <jlayton@redhat.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  fs/iomap/buffered-io.c |  7 -------
> >  fs/xfs/xfs_aops.c      | 17 +++++++++++++++++
> >  fs/xfs/xfs_trans.c     |  3 +++
> >  fs/xfs/xfs_trans.h     | 22 ++++++++++++++++++++++
> >  4 files changed, 42 insertions(+), 7 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.
> > +      */
> > +     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.
> > +      */
> > +     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.c b/fs/xfs/xfs_trans.c
> > index fe20398a214e..08d4916ffb13 100644
> > --- a/fs/xfs/xfs_trans.c
> > +++ b/fs/xfs/xfs_trans.c
> > @@ -124,6 +124,9 @@ xfs_trans_dup(
> >       tp->t_rtx_res = tp->t_rtx_res_used;
> >       ntp->t_pflags = tp->t_pflags;
>
> This one line (ntp->t_pflags = tp->t_pflags) should move to
> xfs_trans_context_swap.
>

Make sense to me.
Will update it.


> --D
>
> >
> > +     /* Associate the new transaction with this thread. */
> > +     xfs_trans_context_swap(tp, ntp);
> > +
> >       /* move deferred ops over to the new tp */
> >       xfs_defer_move(ntp, tp);
> >
> > diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> > index 44b11c64a15e..d596a375e3bf 100644
> > --- a/fs/xfs/xfs_trans.h
> > +++ b/fs/xfs/xfs_trans.h
> > @@ -268,16 +268,38 @@ 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;
> > +}
> > +
> >  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)
> >  {
> > +     ASSERT(current->journal_info == tp);
> > +     current->journal_info = NULL;
> >       memalloc_nofs_restore(tp->t_pflags);
> >  }
> >
> > +/*
> > + * Transfer the transaction context when rolling a permanent
> > + * transaction.
> > + */
> > +static inline void
> > +xfs_trans_context_swap(struct xfs_trans *tp, struct xfs_trans *ntp)
> > +{
> > +     ASSERT(current->journal_info == tp);
> > +     current->journal_info = ntp;
> > +}
> > +
> >  #endif       /* __XFS_TRANS_H__ */
> > --
> > 2.18.4
> >



--
Thanks
Yafang