xfs: create a sparse load xfarray function

Create a new method to load an xfarray element from the xfile, but with
a twist. If we've never stored to the array index, zero the caller's
buffer. This will facilitate RMWs updates of records in a sparse array
without fuss, since the sparse xfarray convention is that uninitialized
array elements default to zeroes.

This is a separate patch to reduce the size of the upcoming quotacheck
patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

xfs: convert xfarray_pagesort to deal with large folios

Convert xfarray_pagesort to handle large folios by introducing a new
xfile_get_folio routine that can return a folio of arbitrary size, and
using heapsort on the full folio. This also corrects an off-by-one bug
in the calculation of len in xfarray_pagesort that was papered over by
xfarray_want_pagesort.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

xfs: remove xfarray_sortinfo.page_kaddr

Now that xfile pages don't need kmapping, there is no need to cache
the kernel virtual address for them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

xfs: repair free space btrees

Rebuild the free space btrees from the gaps in the rmap btree. Refer to
the case study in Documentation/filesystems/xfs-online-fsck-design.rst
for more details.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

xfs: improve xfarray quicksort pivot

Now that we have the means to do insertion sorts of small in-memory
subsets of an xfarray, use it to improve the quicksort pivot algorithm
by reading 7 records into memory and finding the median of that. This
should prevent bad partitioning when a[lo] and a[hi] end up next to each
other in the final sort, which can happen when sorting for cntbt repair
when the free space is extremely fragmented (e.g. generic/176).

This doesn't speed up the average quicksort run by much, but it will
(hopefully) avoid the quadratic time collapse for which quicksort is
famous.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

xfs: speed up xfarray sort by sorting xfile page contents directly

If all the records in an xfarray subset live within the same memory
page, we can short-circuit even more quicksort recursion by mapping that
page into the local CPU and using the kernel's heapsort function to sort
the subset. On the author's computer, this reduces the runtime by
another 15% on a 500,000 element array.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

xfs: convert xfarray insertion sort to heapsort using scratchpad memory

In the previous patch, we created a very basic quicksort implementation
for xfile arrays. While the use of an alternate sorting algorithm to
avoid quicksort recursion on very small subsets reduces the runtime
modestly, we could do better than a load and store-heavy insertion sort,
particularly since each load and store requires a page mapping lookup in
the xfile.

For a small increase in kernel memory requirements, we could instead
bulk load the xfarray records into memory, use the kernel's existing
heapsort implementation to sort the records, and bulk store the memory
buffer back into the xfile. On the author's computer, this reduces the
runtime by about 5% on a 500,000 element array.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

xfs: enable sorting of xfile-backed arrays

The btree bulk loading code requires that records be provided in the
correct record sort order for the given btree type. In general, repair
code cannot be required to collect records in order, and it is not
feasible to insert new records in the middle of an array to maintain
sort order.

Implement a sorting algorithm so that we can sort the records just prior
to bulk loading. In principle, an xfarray could consume many gigabytes
of memory and its backing pages can be sent out to disk at any time.
This means that we cannot map the entire array into memory at once, so
we must find a way to divide the work into smaller portions (e.g. a
page) that /can/ be mapped into memory.

Quicksort seems like a reasonable fit for this purpose, since it uses a
divide and conquer strategy to keep its average runtime logarithmic.
The solution presented here is a port of the glibc implementation, which
itself is derived from the median-of-three and tail call recursion
strategies outlined by Sedgwick.

Subsequent patches will optimize the implementation further by utilizing
the kernel's heapsort on directly-mapped memory whenever possible, and
improving the quicksort pivot selection algorithm to try to avoid O(n^2)
collapses.

Note: The sorting functionality gets its own patch because the basic big
array mechanisms were plenty for a single code patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

xfs: create a big array data structure

Create a simple 'big array' data structure for storage of fixed-size
metadata records that will be used to reconstruct a btree index. For
repair operations, the most important operations are append, iterate,
and sort.

Earlier implementations of the big array used linked lists and suffered
from severe problems -- pinning all records in kernel memory was not a
good idea and frequently lead to OOM situations; random access was very
inefficient; and record overhead for the lists was unacceptably high at
40-60%.

Therefore, the big memory array relies on the 'xfile' abstraction, which
creates a memfd file and stores the records in page cache pages. Since
the memfd is created in tmpfs, the memory pages can be pushed out to
disk if necessary and we have a built-in usage limit of 50% of physical
memory.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Dave Chinner <dchinner@redhat.com>