Re: [PATCH 3/3] xfs: clean up xfs_dialloc() by introducing __xfs_dialloc()

Gao Xiang
2020-11-24T23:14:57Z

Hi Dave,

On Wed, Nov 25, 2020 at 09:16:23AM +1100, Dave Chinner wrote:
> On Tue, Nov 24, 2020 at 11:51:30PM +0800, Gao Xiang wrote:
> > Move the main loop out into a separated function, so we can save
> > many extra xfs_perag_put()s and gotoes to make the logic cleaner.
> >
> > Also it can make the modification of perag protection by some lock
> > for shrinking in the future somewhat easier.
> >
> > Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> > ---
> > It tries to kill multiple goto exits... which makes the logic hard
> > to read and modify.
> >
> > not quite sure the name of __xfs_dialloc(), cannot think of some
> > better name since xfs_dialloc_ag is used...
> >
> > fs/xfs/libxfs/xfs_ialloc.c | 166 ++++++++++++++++++++-----------------
> > 1 file changed, 88 insertions(+), 78 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> > index 5c8b0210aad3..937455c50570 100644
> > --- a/fs/xfs/libxfs/xfs_ialloc.c
> > +++ b/fs/xfs/libxfs/xfs_ialloc.c
> > @@ -1681,6 +1681,83 @@ xfs_dialloc_ag(
> > return error;
> > }
> >
> > +/*
> > + * Return 0 for successfully allocating some inodes in this AG;
> > + * 1 for skipping to allocating in the next AG;
> > + * < 0 for error code.
> > + */
> > +static int
> > +__xfs_dialloc(
>
> FWIW, we try to avoid "__" prefixes when factoring code out. The new
> function should be obvious in it's function to have a proper name.
>
> In this case, we are going to try to allocate an inode in the
> provided AG. So xfs_dialloc_try_ag() would seem appropriate.

Ok, Thanks! I was asking for a real name :)

>
> Also, in that case, an -EAGAIN or -EBUSY error might be more
> appropriate for a "cannot allocate in this AG" soft failure rather
> than having a tri-state return value. We should not get either of
> those errors from inode allocation, so such an error would indicate
> to the caller that is should just try the next AG...

I thought using some error code as well, yet my perference finally
about this could be a functional feature (e.g. non-RCU / completely
retry by using -ECHILD / -ESTALE in lookup code). For such 2 level
internal seperate functions, I'd like to use some tri-state return
value in order to avoid breaking some potential or upcoming functional
features.

Yeah, but it's just my own thought, anyway. I will switch to use
some error code in the next version.

>
> However, I think there's a higher layer cleanup than needs to be
> done here first....
>
> > @@ -1711,7 +1788,6 @@ xfs_dialloc(
> > xfs_ino_t *inop)
> > {
> > struct xfs_mount *mp = tp->t_mountp;
> > - struct xfs_buf *agbp;
> > xfs_agnumber_t agno;
> > int error;
> > bool noroom = false;
> > @@ -1726,8 +1802,9 @@ xfs_dialloc(
> > * continue where we left off before. In this case, we
> > * know that the allocation group has free inodes.
> > */
> > - agbp = *IO_agbp;
> > - goto out_alloc;
> > + error = xfs_dialloc_ag(tp, *IO_agbp, parent, inop);
> > + *IO_agbp = NULL;
> > + return error;
> > }
>
> This whole "ialloc_context/IO_agbp" thing has always been a nasty
> wart in this code. I think getting rid that wart then makes this
> change a lot cleaner and more obvious.
>
> Completely untested patch below for you to start from...

Yeah, I didn't think much more about it to really change the logic.
My starting point was that I always feel a bit more uncomfortable
about randomly add some perag lock/unlock for such like function
(with random multiple exits, and xfs_alloc.c extent allocation
almost the same...). Hard to review and maybe potential unbalanced
bugs here....

Ok, Thanks for the patch/direction! let me play with it for a while,
and if you don't care I'll make it together with the series (although
the whole progress could be somewhat slow though...)

Thanks,
Gao Xiang

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
>
>
> xfs: rework inode allocation interface
>
> From: Dave Chinner <dchinner@redhat.com>
>
> Get rid of the nasty, confusing ialloc_context and messy ENOSPC and
> failure handling around xfs_ialloc() and xfs_dialloc() by separating
> free inode chunk allocation and inode allocation into two individual
> high level operations.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_ialloc.c | 132 +++++++++++++-----------
> fs/xfs/libxfs/xfs_ialloc.h | 42 ++++----
> fs/xfs/xfs_inode.c | 245 +++++++++++----------------------------------
> 3 files changed, 151 insertions(+), 268 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 974e71bc4a3a..011813934ee1 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -1570,7 +1570,7 @@ xfs_dialloc_ag_update_inobt(
> * The caller selected an AG for us, and made sure that free inodes are
> * available.
> */
> -STATIC int
> +int
> xfs_dialloc_ag(
> struct xfs_trans *tp,
> struct xfs_buf *agbp,
> @@ -1682,35 +1682,70 @@ xfs_dialloc_ag(
> return error;
> }
>
> +static int
> +xfs_dialloc_roll(
> + struct xfs_trans **tpp,
> + struct xfs_buf *agibp)
> +{
> + struct xfs_trans *tp = *tpp;
> + void *dqinfo = NULL;
> + unsigned int tflags = 0;
> + int error;
> +
> + /*
> + * Hold to on to the agibp across the commit so no other allocation can
> + * come in and take the free inodes we just allocated for our caller.
> + */
> + xfs_trans_bhold(tp, agibp);
> +
> + /*
> + * We want the quota changes to be associated with the next transaction,
> + * NOT this one. So, detach the dqinfo from this and attach it to the
> + * next transaction.
> + */
> + if (tp->t_dqinfo) {
> + dqinfo = tp->t_dqinfo;
> + tp->t_dqinfo = NULL;
> + tflags = tp->t_flags & XFS_TRANS_DQ_DIRTY;
> + tp->t_flags &= ~(XFS_TRANS_DQ_DIRTY);
> + }
> +
> + error = xfs_trans_roll(&tp);
> +
> + /* Re-attach the quota info that we detached from prev trx. */
> + if (dqinfo) {
> + tp->t_dqinfo = dqinfo;
> + tp->t_flags |= tflags;
> + }
> +
> + *tpp = tp;
> + if (error) {
> + xfs_buf_relse(agibp);
> + return error;
> + }
> + xfs_trans_bjoin(tp, agibp);
> + return 0;
> +}
> +
> /*
> - * Allocate an inode on disk.
> + * Select and prepare an AG for inode allocation.
> *
> - * Mode is used to tell whether the new inode will need space, and whether it
> - * is a directory.
> + * Mode is used to tell whether the new inode is a directory and hence where to
> + * locate it.
> *
> - * This function is designed to be called twice if it has to do an allocation
> - * to make more free inodes. On the first call, *IO_agbp should be set to NULL.
> - * If an inode is available without having to performn an allocation, an inode
> - * number is returned. In this case, *IO_agbp is set to NULL. If an allocation
> - * needs to be done, xfs_dialloc returns the current AGI buffer in *IO_agbp.
> - * The caller should then commit the current transaction, allocate a
> - * new transaction, and call xfs_dialloc() again, passing in the previous value
> - * of *IO_agbp. IO_agbp should be held across the transactions. Since the AGI
> - * buffer is locked across the two calls, the second call is guaranteed to have
> - * a free inode available.
> - *
> - * Once we successfully pick an inode its number is returned and the on-disk
> - * data structures are updated. The inode itself is not read in, since doing so
> - * would break ordering constraints with xfs_reclaim.
> + * This function will ensure that the selected AG has free inodes available to
> + * allocate from. The selected AGI will be returned locked to the caller, and it
> + * will allocate more free inodes if required. If no free inodes are found or
> + * can be allocated, no AGI will be returned.
> */
> int
> -xfs_dialloc(
> - struct xfs_trans *tp,
> +xfs_dialloc_select_ag(
> + struct xfs_trans **tpp,
> xfs_ino_t parent,
> umode_t mode,
> - struct xfs_buf **IO_agbp,
> - xfs_ino_t *inop)
> + struct xfs_buf **IO_agbp)
> {
> + struct xfs_trans *tp = *tpp;
> struct xfs_mount *mp = tp->t_mountp;
> struct xfs_buf *agbp;
> xfs_agnumber_t agno;
> @@ -1722,25 +1757,10 @@ xfs_dialloc(
> struct xfs_ino_geometry *igeo = M_IGEO(mp);
> int okalloc = 1;
>
> - if (*IO_agbp) {
> - /*
> - * If the caller passes in a pointer to the AGI buffer,
> - * continue where we left off before. In this case, we
> - * know that the allocation group has free inodes.
> - */
> - agbp = *IO_agbp;
> - goto out_alloc;
> - }
> -
> - /*
> - * We do not have an agbp, so select an initial allocation
> - * group for inode allocation.
> - */
> + *IO_agbp = NULL;
> start_agno = xfs_ialloc_ag_select(tp, parent, mode);
> - if (start_agno == NULLAGNUMBER) {
> - *inop = NULLFSINO;
> + if (start_agno == NULLAGNUMBER)
> return 0;
> - }
>
> /*
> * If we have already hit the ceiling of inode blocks then clear
> @@ -1773,7 +1793,7 @@ xfs_dialloc(
> if (!pag->pagi_init) {
> error = xfs_ialloc_pagi_init(mp, tp, agno);
> if (error)
> - goto out_error;
> + break;
> }
>
> /*
> @@ -1788,11 +1808,12 @@ xfs_dialloc(
> */
> error = xfs_ialloc_read_agi(mp, tp, agno, &agbp);
> if (error)
> - goto out_error;
> + break;
>
> if (pag->pagi_freecount) {
> xfs_perag_put(pag);
> - goto out_alloc;
> + *IO_agbp = agbp;
> + return 0;
> }
>
> if (!okalloc)
> @@ -1802,46 +1823,39 @@ xfs_dialloc(
> error = xfs_ialloc_ag_alloc(tp, agbp, &ialloced);
> if (error) {
> xfs_trans_brelse(tp, agbp);
> -
> if (error != -ENOSPC)
> - goto out_error;
> -
> - xfs_perag_put(pag);
> - *inop = NULLFSINO;
> - return 0;
> + break;
> }
>
> if (ialloced) {
> /*
> - * We successfully allocated some inodes, return
> - * the current context to the caller so that it
> - * can commit the current transaction and call
> - * us again where we left off.
> + * We successfully allocated some inodes, so roll the
> + * transaction and return the locked AGI buffer to the
> + * caller so they can allocate one of the free inodes we
> + * just prepared for them.
> */
> ASSERT(pag->pagi_freecount > 0);
> xfs_perag_put(pag);
>
> + error = xfs_dialloc_roll(&tp, agbp);
> + *tpp = tp;
> + if (error)
> + return error;
> *IO_agbp = agbp;
> - *inop = NULLFSINO;
> return 0;
> }
>
> nextag_relse_buffer:
> - xfs_trans_brelse(tp, agbp);
> + xfs_trans_brelse(*tpp, agbp);
> nextag:
> xfs_perag_put(pag);
> if (++agno == mp->m_sb.sb_agcount)
> agno = 0;
> if (agno == start_agno) {
> - *inop = NULLFSINO;
> return noroom ? -ENOSPC : 0;
> }
> }
>
> -out_alloc:
> - *IO_agbp = NULL;
> - return xfs_dialloc_ag(tp, agbp, parent, inop);
> -out_error:
> xfs_perag_put(pag);
> return error;
> }
> diff --git a/fs/xfs/libxfs/xfs_ialloc.h b/fs/xfs/libxfs/xfs_ialloc.h
> index 72b3468b97b1..d8de4b9f6603 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.h
> +++ b/fs/xfs/libxfs/xfs_ialloc.h
> @@ -37,30 +37,26 @@ xfs_make_iptr(struct xfs_mount *mp, struct xfs_buf *b, int o)
> * Mode is used to tell whether the new inode will need space, and whether
> * it is a directory.
> *
> - * To work within the constraint of one allocation per transaction,
> - * xfs_dialloc() is designed to be called twice if it has to do an
> - * allocation to make more free inodes. If an inode is
> - * available without an allocation, agbp would be set to the current
> - * agbp and alloc_done set to false.
> - * If an allocation needed to be done, agbp would be set to the
> - * inode header of the allocation group and alloc_done set to true.
> - * The caller should then commit the current transaction and allocate a new
> - * transaction. xfs_dialloc() should then be called again with
> - * the agbp value returned from the previous call.
> - *
> - * Once we successfully pick an inode its number is returned and the
> - * on-disk data structures are updated. The inode itself is not read
> - * in, since doing so would break ordering constraints with xfs_reclaim.
> - *
> - * *agbp should be set to NULL on the first call, *alloc_done set to FALSE.
> + * There are two phases to inode allocation: selecting an AG and ensuring
> + * that it contains free inodes, followed by allocating one of the free
> + * inodes. xfs_dialloc_select_ag() does the former and returns a locked AGI
> + * to the caller, ensuring that followup call to xfs_dialloc_ag() will
> + * have free inodes to allocate from. xfs_dialloc_ag() will return the inode
> + * number of the free inode we allocated.
> */
> -int /* error */
> -xfs_dialloc(
> - struct xfs_trans *tp, /* transaction pointer */
> - xfs_ino_t parent, /* parent inode (directory) */
> - umode_t mode, /* mode bits for new inode */
> - struct xfs_buf **agbp, /* buf for a.g. inode header */
> - xfs_ino_t *inop); /* inode number allocated */
> +int
> +xfs_dialloc_select_ag(
> + struct xfs_trans **tpp,
> + xfs_ino_t parent,
> + umode_t mode,
> + struct xfs_buf **agibp);
> +
> +int
> +xfs_dialloc_ag(
> + struct xfs_trans *tp,
> + struct xfs_buf *agbp,
> + xfs_ino_t parent,
> + xfs_ino_t *inop);
>
> /*
> * Free disk inode. Carefully avoids touching the incore inode, all
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 2bfbcf28b1bd..2a06ca4dee2b 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -761,68 +761,28 @@ xfs_inode_inherit_flags2(
> }
>
> /*
> - * Allocate an inode on disk and return a copy of its in-core version.
> - * The in-core inode is locked exclusively. Set mode, nlink, and rdev
> - * appropriately within the inode. The uid and gid for the inode are
> - * set according to the contents of the given cred structure.
> - *
> - * Use xfs_dialloc() to allocate the on-disk inode. If xfs_dialloc()
> - * has a free inode available, call xfs_iget() to obtain the in-core
> - * version of the allocated inode. Finally, fill in the inode and
> - * log its initial contents. In this case, ialloc_context would be
> - * set to NULL.
> - *
> - * If xfs_dialloc() does not have an available inode, it will replenish
> - * its supply by doing an allocation. Since we can only do one
> - * allocation within a transaction without deadlocks, we must commit
> - * the current transaction before returning the inode itself.
> - * In this case, therefore, we will set ialloc_context and return.
> - * The caller should then commit the current transaction, start a new
> - * transaction, and call xfs_ialloc() again to actually get the inode.
> - *
> - * To ensure that some other process does not grab the inode that
> - * was allocated during the first call to xfs_ialloc(), this routine
> - * also returns the [locked] bp pointing to the head of the freelist
> - * as ialloc_context. The caller should hold this buffer across
> - * the commit and pass it back into this routine on the second call.
> - *
> - * If we are allocating quota inodes, we do not have a parent inode
> - * to attach to or associate with (i.e. pip == NULL) because they
> - * are not linked into the directory structure - they are attached
> - * directly to the superblock - and so have no parent.
> + * Initialise a newly allocated inode and return the in-core inode to the
> + * caller locked exclusively.
> */
> static int
> xfs_ialloc(
> - xfs_trans_t *tp,
> - xfs_inode_t *pip,
> - umode_t mode,
> - xfs_nlink_t nlink,
> - dev_t rdev,
> - prid_t prid,
> - xfs_buf_t **ialloc_context,
> - xfs_inode_t **ipp)
> -{
> - struct xfs_mount *mp = tp->t_mountp;
> - xfs_ino_t ino;
> - xfs_inode_t *ip;
> - uint flags;
> - int error;
> - struct timespec64 tv;
> - struct inode *inode;
> + struct xfs_trans *tp,
> + struct xfs_inode *pip,
> + umode_t mode,
> + xfs_nlink_t nlink,
> + dev_t rdev,
> + prid_t prid,
> + struct xfs_inode **ipp)
> +{
> + struct xfs_mount *mp = tp->t_mountp;
> + struct xfs_inode *ip;
> + xfs_ino_t ino;
> + uint flags;
> + int error;
> + struct timespec64 tv;
> + struct inode *inode;
>
> - /*
> - * Call the space management code to pick
> - * the on-disk inode to be allocated.
> - */
> - error = xfs_dialloc(tp, pip ? pip->i_ino : 0, mode,
> - ialloc_context, &ino);
> - if (error)
> - return error;
> - if (*ialloc_context || ino == NULLFSINO) {
> - *ipp = NULL;
> - return 0;
> - }
> - ASSERT(*ialloc_context == NULL);
> + *ipp = NULL;
>
> /*
> * Protect against obviously corrupt allocation btree records. Later
> @@ -837,12 +797,10 @@ xfs_ialloc(
> }
>
> /*
> - * Get the in-core inode with the lock held exclusively.
> - * This is because we're setting fields here we need
> - * to prevent others from looking at until we're done.
> + * Get the in-core inode with the lock held exclusively to prevent
> + * others from looking at until we're done.
> */
> - error = xfs_iget(mp, tp, ino, XFS_IGET_CREATE,
> - XFS_ILOCK_EXCL, &ip);
> + error = xfs_iget(mp, tp, ino, XFS_IGET_CREATE, XFS_ILOCK_EXCL, &ip);
> if (error)
> return error;
> ASSERT(ip != NULL);
> @@ -932,142 +890,57 @@ xfs_ialloc(
> }
>
> /*
> - * Allocates a new inode from disk and return a pointer to the
> - * incore copy. This routine will internally commit the current
> - * transaction and allocate a new one if the Space Manager needed
> - * to do an allocation to replenish the inode free-list.
> - *
> - * This routine is designed to be called from xfs_create and
> - * xfs_create_dir.
> + * Allocates a new inode from disk and return a pointer to the incore copy. This
> + * routine will internally commit the current transaction and allocate a new one
> + * if we needed to allocate more on-disk free inodes to perform the requested
> + * operation.
> *
> + * If we are allocating quota inodes, we do not have a parent inode to attach to
> + * or associate with (i.e. dp == NULL) because they are not linked into the
> + * directory structure - they are attached directly to the superblock - and so
> + * have no parent.
> */
> int
> xfs_dir_ialloc(
> - xfs_trans_t **tpp, /* input: current transaction;
> - output: may be a new transaction. */
> - xfs_inode_t *dp, /* directory within whose allocate
> - the inode. */
> - umode_t mode,
> - xfs_nlink_t nlink,
> - dev_t rdev,
> - prid_t prid, /* project id */
> - xfs_inode_t **ipp) /* pointer to inode; it will be
> - locked. */
> -{
> - xfs_trans_t *tp;
> - xfs_inode_t *ip;
> - xfs_buf_t *ialloc_context = NULL;
> - int code;
> - void *dqinfo;
> - uint tflags;
> -
> - tp = *tpp;
> - ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
> + struct xfs_trans **tpp,
> + struct xfs_inode *dp,
> + umode_t mode,
> + xfs_nlink_t nlink,
> + dev_t rdev,
> + prid_t prid,
> + struct xfs_inode **ipp)
> +{
> + struct xfs_buf *agibp = NULL;
> + xfs_ino_t pino = dp ? dp->i_ino : 0;
> + xfs_ino_t ino;
> + int error;
>
> - /*
> - * xfs_ialloc will return a pointer to an incore inode if
> - * the Space Manager has an available inode on the free
> - * list. Otherwise, it will do an allocation and replenish
> - * the freelist. Since we can only do one allocation per
> - * transaction without deadlocks, we will need to commit the
> - * current transaction and start a new one. We will then
> - * need to call xfs_ialloc again to get the inode.
> - *
> - * If xfs_ialloc did an allocation to replenish the freelist,
> - * it returns the bp containing the head of the freelist as
> - * ialloc_context. We will hold a lock on it across the
> - * transaction commit so that no other process can steal
> - * the inode(s) that we've just allocated.
> - */
> - code = xfs_ialloc(tp, dp, mode, nlink, rdev, prid, &ialloc_context,
> - &ip);
> + ASSERT(*tpp->t_flags & XFS_TRANS_PERM_LOG_RES);
>
> /*
> - * Return an error if we were unable to allocate a new inode.
> - * This should only happen if we run out of space on disk or
> - * encounter a disk error.
> + * Call the space management code to pick the on-disk inode to be
> + * allocated.
> */
> - if (code) {
> - *ipp = NULL;
> - return code;
> - }
> - if (!ialloc_context && !ip) {
> + error = xfs_dialloc_select_ag(tpp, pino, mode, &agibp);
> + if (error)
> + return error;
> + if (!agibp) {
> *ipp = NULL;
> - return -ENOSPC;
> + return 0;
> }
>
> - /*
> - * If the AGI buffer is non-NULL, then we were unable to get an
> - * inode in one operation. We need to commit the current
> - * transaction and call xfs_ialloc() again. It is guaranteed
> - * to succeed the second time.
> - */
> - if (ialloc_context) {
> - /*
> - * Normally, xfs_trans_commit releases all the locks.
> - * We call bhold to hang on to the ialloc_context across
> - * the commit. Holding this buffer prevents any other
> - * processes from doing any allocations in this
> - * allocation group.
> - */
> - xfs_trans_bhold(tp, ialloc_context);
> -
> - /*
> - * We want the quota changes to be associated with the next
> - * transaction, NOT this one. So, detach the dqinfo from this
> - * and attach it to the next transaction.
> - */
> - dqinfo = NULL;
> - tflags = 0;
> - if (tp->t_dqinfo) {
> - dqinfo = (void *)tp->t_dqinfo;
> - tp->t_dqinfo = NULL;
> - tflags = tp->t_flags & XFS_TRANS_DQ_DIRTY;
> - tp->t_flags &= ~(XFS_TRANS_DQ_DIRTY);
> - }
> -
> - code = xfs_trans_roll(&tp);
> -
> - /*
> - * Re-attach the quota info that we detached from prev trx.
> - */
> - if (dqinfo) {
> - tp->t_dqinfo = dqinfo;
> - tp->t_flags |= tflags;
> - }
> -
> - if (code) {
> - xfs_buf_relse(ialloc_context);
> - *tpp = tp;
> - *ipp = NULL;
> - return code;
> - }
> - xfs_trans_bjoin(tp, ialloc_context);
> -
> - /*
> - * Call ialloc again. Since we've locked out all
> - * other allocations in this allocation group,
> - * this call should always succeed.
> - */
> - code = xfs_ialloc(tp, dp, mode, nlink, rdev, prid,
> - &ialloc_context, &ip);
> -
> - /*
> - * If we get an error at this point, return to the caller
> - * so that the current transaction can be aborted.
> - */
> - if (code) {
> - *tpp = tp;
> - *ipp = NULL;
> - return code;
> - }
> - ASSERT(!ialloc_context && ip);
> + /* Allocate an inode from the selected AG */
> + error = xfs_dialloc_ag(*tpp, agibp, pino, &ino);
> + if (error)
> + return error;
> + ASSERT(ino != NULLFSINO);
>
> + /* Initialise the newly allocated inode. */
> + error = xfs_ialloc(*tpp, dp, mode, nlink, rdev, prid, ipp);
> + if (error) {
> + *ipp = NULL;
> + return error;
> }
> -
> - *ipp = ip;
> - *tpp = tp;
> -
> return 0;
> }
>
>