Re: [PATCH 17/42] xfs: combine __xfs_alloc_vextent_this_ag and xfs_alloc_ag_vextent

Darrick J. Wong
2023-02-01T22:25:41Z

On Thu, Jan 19, 2023 at 09:44:40AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> There's a bit of a recursive conundrum around
> xfs_alloc_ag_vextent(). We can't first call xfs_alloc_ag_vextent()
> without preparing the AGFL for the allocation, and preparing the
> AGFL calls xfs_alloc_ag_vextent() to prepare the AGFL for the
> allocation. This "double allocation" requirement is not really clear
> from the current xfs_alloc_fix_freelist() calls that are sprinkled
> through the allocation code.
>
> It's not helped that xfs_alloc_ag_vextent() can actually allocate
> from the AGFL itself, but there's special code to prevent AGFL prep
> allocations from allocating from the free list it's trying to prep.
> The naming is also not consistent: args->wasfromfl is true when we
> allocated _from_ the free list, but the indication that we are
> allocating _for_ the free list is via checking that (args->resv ==
> XFS_AG_RESV_AGFL).
>
> So, lets make this "allocation required for allocation" situation
> clear by moving it all inside xfs_alloc_ag_vextent(). The freelist
> allocation is a specific XFS_ALLOCTYPE_THIS_AG allocation, which
> translated directly to xfs_alloc_ag_vextent_size() allocation.
>
> This enables us to replace __xfs_alloc_vextent_this_ag() with a call
> to xfs_alloc_ag_vextent(), and we drive the freelist fixing further
> into the per-ag allocation algorithm.

Hmm. My first reaction to all this was "why do I care about all this
slicing and dicing?" and "uugh what confuusing". Then I skipped to the
end of the book and observed that the end goal seems to be the
elimination of:

args.type = XFS_ALLOC_TYPE_START_AG;
args.fsbno = sometarget;
/* fill out other fields mysteriously */

by turning them all into explicit functions!

error = xfs_alloc_vextent_start_ag(&args, sometarget);

So I looked at all the replacements and noticed that it's quite a bit
easier to understand what each variant on allocation does.

It took me a minute to realize that the additional call to
xfs_rmap_should_skip_owner_update is because _alloc_fix_freelist doesn't
call what becomes the _vextent_finish function.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_alloc.c | 65 +++++++++++++++++++++------------------
> 1 file changed, 35 insertions(+), 30 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 2dec95f35562..011baace7e9d 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -1140,22 +1140,38 @@ xfs_alloc_ag_vextent_small(
> * and of the form k * prod + mod unless there's nothing that large.
> * Return the starting a.g. block, or NULLAGBLOCK if we can't do it.
> */
> -STATIC int /* error */
> +static int
> xfs_alloc_ag_vextent(
> - xfs_alloc_arg_t *args) /* argument structure for allocation */
> + struct xfs_alloc_arg *args)
> {
> - int error=0;
> + struct xfs_mount *mp = args->mp;
> + int error = 0;
>
> ASSERT(args->minlen > 0);
> ASSERT(args->maxlen > 0);
> ASSERT(args->minlen <= args->maxlen);
> ASSERT(args->mod < args->prod);
> ASSERT(args->alignment > 0);
> + ASSERT(args->resv != XFS_AG_RESV_AGFL);
> +
> +
> + error = xfs_alloc_fix_freelist(args, 0);
> + if (error) {
> + trace_xfs_alloc_vextent_nofix(args);
> + return error;
> + }
> + if (!args->agbp) {
> + /* cannot allocate in this AG at all */
> + trace_xfs_alloc_vextent_noagbp(args);
> + args->agbno = NULLAGBLOCK;
> + return 0;
> + }
> + args->agbno = XFS_FSB_TO_AGBNO(mp, args->fsbno);
> + args->wasfromfl = 0;
>
> /*
> * Branch to correct routine based on the type.
> */
> - args->wasfromfl = 0;
> switch (args->type) {
> case XFS_ALLOCTYPE_THIS_AG:
> error = xfs_alloc_ag_vextent_size(args);
> @@ -1176,7 +1192,6 @@ xfs_alloc_ag_vextent(
>
> ASSERT(args->len >= args->minlen);
> ASSERT(args->len <= args->maxlen);
> - ASSERT(!args->wasfromfl || args->resv != XFS_AG_RESV_AGFL);
> ASSERT(args->agbno % args->alignment == 0);
>
> /* if not file data, insert new block into the reverse map btree */
> @@ -2721,7 +2736,7 @@ xfs_alloc_fix_freelist(
> targs.resv = XFS_AG_RESV_AGFL;
>
> /* Allocate as many blocks as possible at once. */
> - error = xfs_alloc_ag_vextent(&targs);
> + error = xfs_alloc_ag_vextent_size(&targs);
> if (error)
> goto out_agflbp_relse;
>
> @@ -2735,6 +2750,18 @@ xfs_alloc_fix_freelist(
> break;
> goto out_agflbp_relse;
> }
> +
> + if (!xfs_rmap_should_skip_owner_update(&targs.oinfo)) {
> + error = xfs_rmap_alloc(tp, agbp, pag,
> + targs.agbno, targs.len, &targs.oinfo);
> + if (error)
> + goto out_agflbp_relse;
> + }
> + error = xfs_alloc_update_counters(tp, agbp,
> + -((long)(targs.len)));
> + if (error)
> + goto out_agflbp_relse;
> +
> /*
> * Put each allocated block on the list.
> */
> @@ -3244,28 +3271,6 @@ xfs_alloc_vextent_set_fsbno(
> /*
> * Allocate within a single AG only.
> */
> -static int
> -__xfs_alloc_vextent_this_ag(
> - struct xfs_alloc_arg *args)
> -{
> - struct xfs_mount *mp = args->mp;
> - int error;
> -
> - error = xfs_alloc_fix_freelist(args, 0);
> - if (error) {
> - trace_xfs_alloc_vextent_nofix(args);
> - return error;
> - }
> - if (!args->agbp) {
> - /* cannot allocate in this AG at all */
> - trace_xfs_alloc_vextent_noagbp(args);
> - args->agbno = NULLAGBLOCK;
> - return 0;
> - }
> - args->agbno = XFS_FSB_TO_AGBNO(mp, args->fsbno);
> - return xfs_alloc_ag_vextent(args);
> -}
> -
> static int
> xfs_alloc_vextent_this_ag(
> struct xfs_alloc_arg *args,
> @@ -3289,7 +3294,7 @@ xfs_alloc_vextent_this_ag(
> }
>
> args->pag = xfs_perag_get(mp, args->agno);
> - error = __xfs_alloc_vextent_this_ag(args);
> + error = xfs_alloc_ag_vextent(args);
>
> xfs_alloc_vextent_set_fsbno(args, minimum_agno);
> xfs_perag_put(args->pag);
> @@ -3329,7 +3334,7 @@ xfs_alloc_vextent_iterate_ags(
> args->agno = start_agno;
> for (;;) {
> args->pag = xfs_perag_get(mp, args->agno);
> - error = __xfs_alloc_vextent_this_ag(args);
> + error = xfs_alloc_ag_vextent(args);
> if (error) {
> args->agbno = NULLAGBLOCK;
> break;
> --
> 2.39.0
>