linux.git/Documentation/core-api/xarray.rst, branch v6.18.21

xarray: add xas_try_split() to split a multi-index entry

2025-03-18T05:06:59+00:00

Patch series "Buddy allocator like (or non-uniform) folio split", v10.

This patchset adds a new buddy allocator like (or non-uniform) large folio
split from a order-n folio to order-m with m < n.  It reduces

1. the total number of after-split folios from 2^(n-m) to n-m+1;

2. the amount of memory needed for multi-index xarray split from 2^(n/6-m/6) to
   n/6-m/6, assuming XA_CHUNK_SHIFT=6;

3. keep more large folios after a split from all order-m folios to
   order-(n-1) to order-m folios.

For example, to split an order-9 to order-0, folio split generates 10 (or
11 for anonymous memory) folios instead of 512, allocates 1 xa_node
instead of 8, and leaves 1 order-8, 1 order-7, ..., 1 order-1 and 2
order-0 folios (or 4 order-0 for anonymous memory) instead of 512 order-0
folios.

Instead of duplicating existing split_huge_page*() code, __folio_split()
is introduced as the shared backend code for both
split_huge_page_to_list_to_order() and folio_split().  __folio_split() can
support both uniform split and buddy allocator like (or non-uniform)
split.  All existing split_huge_page*() users can be gradually converted
to use folio_split() if possible.  In this patchset, I converted
truncate_inode_partial_folio() to use folio_split().

xfstests quick group passed for both tmpfs and xfs.  I also
semi-replicated Hugh's test[12] and ran it without any issue for almost 24
hours.


This patch (of 8):

A preparation patch for non-uniform folio split, which always split a
folio into half iteratively, and minimal xarray entry split.

Currently, xas_split_alloc() and xas_split() always split all slots from a
multi-index entry.  They cost the same number of xa_node as the
to-be-split slots.  For example, to split an order-9 entry, which takes
2^(9-6)=8 slots, assuming XA_CHUNK_SHIFT is 6 (!CONFIG_BASE_SMALL), 8
xa_node are needed.  Instead xas_try_split() is intended to be used
iteratively to split the order-9 entry into 2 order-8 entries, then split
one order-8 entry, based on the given index, to 2 order-7 entries, ...,
and split one order-1 entry to 2 order-0 entries.  When splitting the
order-6 entry and a new xa_node is needed, xas_try_split() will try to
allocate one if possible.  As a result, xas_try_split() would only need 1
xa_node instead of 8.

When a new xa_node is needed during the split, xas_try_split() can try to
allocate one but no more.  -ENOMEM will be return if a node cannot be
allocated.  -EINVAL will be return if a sibling node is split or cascade
split happens, where two or more new nodes are needed, and these are not
supported by xas_try_split().

xas_split_alloc() and xas_split() split an order-9 to order-0:

         ---------------------------------
         |   |   |   |   |   |   |   |   |
         | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
         |   |   |   |   |   |   |   |   |
         ---------------------------------
           |   |                   |   |
     -------   ---               ---   -------
     |           |     ...       |           |
     V           V               V           V
----------- -----------     ----------- -----------
| xa_node | | xa_node | ... | xa_node | | xa_node |
----------- -----------     ----------- -----------

xas_try_split() splits an order-9 to order-0:
   ---------------------------------
   |   |   |   |   |   |   |   |   |
   | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
   |   |   |   |   |   |   |   |   |
   ---------------------------------
     |
     |
     V
-----------
| xa_node |
-----------

Link: https://lkml.kernel.org/r/20250307174001.242794-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250307174001.242794-2-ziy@nvidia.com
Signed-off-by: Zi Yan 
Cc: Baolin Wang 
Cc: David Hildenbrand 
Cc: Hugh Dickins 
Cc: John Hubbard 
Cc: Kefeng Wang 
Cc: Kirill A. Shuemov 
Cc: Miaohe Lin 
Cc: Matthew Wilcox 
Cc: Ryan Roberts 
Cc: Yang Shi 
Cc: Yu Zhao 
Cc: Zi Yan 
Cc: Kairui Song 
Signed-off-by: Andrew Morton

XArray: minor documentation improvements

2025-01-13T04:21:08+00:00

- Replace "they" with "you" where "you" is used in the preceding
  sentence fragment.
- Mention `xa_erase` in discussion of multi-index entries.  Split this
  into a separate sentence.
- Add "call" parentheses on "xa_store" for consistency and
  linkification.
- Add caveat that `xa_store` and `xa_erase` are not equivalent in the
  presence of `XA_FLAGS_ALLOC`.

Link: https://lkml.kernel.org/r/20241105-xarray-documentation-v5-1-8e1702321b41@gmail.com
Signed-off-by: Tamir Duberstein 
Acked-by: Randy Dunlap 
Reviewed-by: Bagas Sanjaya 
Cc: Jonathan Corbet 
Cc: Matthew Wilcox 
Signed-off-by: Andrew Morton

XArray: Document the locking requirement for the xa_state

2022-02-03T20:56:50+00:00

It wasn't obvious to all readers that it's unsafe to reuse an xa_state
after dropping the xas_lock() or the rcu_read_lock().

Reported-by: Charan Teja Kalla 
Signed-off-by: Matthew Wilcox (Oracle)

XArray: add xas_split

2020-10-16T18:11:15+00:00

In order to use multi-index entries for huge pages in the page cache, we
need to be able to split a multi-index entry (eg if a file is truncated in
the middle of a huge page entry).  This version does not support splitting
more than one level of the tree at a time.  This is an acceptable
limitation for the page cache as we do not expect to support order-12
pages in the near future.

[akpm@linux-foundation.org: export xas_split_alloc() to modules]
[willy@infradead.org: fix xarray split]
  Link: https://lkml.kernel.org/r/20200910175450.GV6583@casper.infradead.org
[willy@infradead.org: fix xarray]
  Link: https://lkml.kernel.org/r/20201001233943.GW20115@casper.infradead.org

Signed-off-by: Matthew Wilcox (Oracle) 
Signed-off-by: Andrew Morton 
Cc: "Kirill A . Shutemov" 
Cc: Qian Cai 
Cc: Song Liu 
Link: https://lkml.kernel.org/r/20200903183029.14930-3-willy@infradead.org
Signed-off-by: Linus Torvalds

XArray: Add xa_for_each_range

2020-01-18T03:33:37+00:00

This function supports iterating over a range of an array.  Also add
documentation links for xa_for_each_start().

Signed-off-by: Matthew Wilcox (Oracle)

XArray: Improve documentation of search marks

2019-11-09T04:48:40+00:00

Move most of the mark-related documentation to its own section to make
it easier to understand.  Add clarification that you can't search for
an unset mark, and you can't yet search for combinations of marks.

Signed-off-by: Matthew Wilcox (Oracle)

docs: remove :c:func: annotations from xarray.rst

2019-06-26T17:14:15+00:00

Now that the build system automatically marks up function references, we
don't have to clutter the source files, so take it out.

[Some paragraphs could now benefit from refilling, but that was left out to
avoid obscuring the real changes.]

Acked-by: Matthew Wilcox 
Signed-off-by: Jonathan Corbet

XArray: Use xa_cmpxchg to implement xa_reserve

2019-02-20T22:08:54+00:00

Jason feels this is clearer, and it saves a function and an exported
symbol.

Suggested-by: Jason Gunthorpe 
Signed-off-by: Matthew Wilcox

XArray: Add cyclic allocation

2019-02-06T18:32:25+00:00

This differs slightly from the IDR equivalent in five ways.

1. It can allocate up to UINT_MAX instead of being limited to INT_MAX,
   like xa_alloc().  Also like xa_alloc(), it will write to the 'id'
   pointer before placing the entry in the XArray.
2. The 'next' cursor is allocated separately from the XArray instead
   of being part of the IDR.  This saves memory for all the users which
   do not use the cyclic allocation API and suits some users better.
3. It returns -EBUSY instead of -ENOSPC.
4. It will attempt to wrap back to the minimum value on memory allocation
   failure as well as on an -EBUSY error, assuming that a user would
   rather allocate a small ID than suffer an ID allocation failure.
5. It reports whether it has wrapped, which is important to some users.

Signed-off-by: Matthew Wilcox

XArray: Add support for 1s-based allocation

2019-02-06T18:13:24+00:00

A lot of places want to allocate IDs starting at 1 instead of 0.
While the xa_alloc() API supports this, it's not very efficient if lots
of IDs are allocated, due to having to walk down to the bottom of the
tree to see if ID 1 is available, then all the way over to the next
non-allocated ID.  This method marks ID 0 as being occupied which wastes
one slot in the XArray, but preserves xa_empty() as working.

Signed-off-by: Matthew Wilcox