[PATCH v8 03/10] readahead: allocate folios with mapping_min_order in readahead

Pankaj Raghav (Samsung)
2024-06-25T11:44:43Z

From: Pankaj Raghav <p.raghav@samsung.com>

page_cache_ra_unbounded() was allocating single pages (0 order folios)
if there was no folio found in an index. Allocate mapping_min_order folios
as we need to guarantee the minimum order if it is set.
While we are at it, rework the loop in page_cache_ra_unbounded() to
advance with the number of pages in a folio instead of just one page at
a time.

page_cache_ra_order() tries to allocate folio to the page cache with a
higher order if the index aligns with that order. Modify it so that the
order does not go below the mapping_min_order requirement of the page
cache. This function will do the right thing even if the new_order passed
is less than the mapping_min_order.
When adding new folios to the page cache we must also ensure the index
used is aligned to the mapping_min_order as the page cache requires the
index to be aligned to the order of the folio.

readahead_expand() is called from readahead aops to extend the range of
the readahead so this function can assume ractl->_index to be aligned with
min_order.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Co-developed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
---
mm/readahead.c | 81 +++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 63 insertions(+), 18 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 66058ae02f2e..2acfd6447d7b 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -206,9 +206,10 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
unsigned long nr_to_read, unsigned long lookahead_size)
{
struct address_space *mapping = ractl->mapping;
- unsigned long index = readahead_index(ractl);
+ unsigned long ra_folio_index, index = readahead_index(ractl);
gfp_t gfp_mask = readahead_gfp_mask(mapping);
- unsigned long i;
+ unsigned long mark, i = 0;
+ unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);

/*
* Partway through the readahead operation, we will have added
@@ -223,10 +224,26 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
unsigned int nofs = memalloc_nofs_save();

filemap_invalidate_lock_shared(mapping);
+ index = mapping_align_index(mapping, index);
+
+ /*
+ * As iterator `i` is aligned to min_nrpages, round_up the
+ * difference between nr_to_read and lookahead_size to mark the
+ * index that only has lookahead or "async_region" to set the
+ * readahead flag.
+ */
+ ra_folio_index = round_up(readahead_index(ractl) + nr_to_read - lookahead_size,
+ min_nrpages);
+ mark = ra_folio_index - index;
+ if (index != readahead_index(ractl)) {
+ nr_to_read += readahead_index(ractl) - index;
+ ractl->_index = index;
+ }
+
/*
* Preallocate as many pages as we will need.
*/
- for (i = 0; i < nr_to_read; i++) {
+ while (i < nr_to_read) {
struct folio *folio = xa_load(&mapping->i_pages, index + i);
int ret;

@@ -240,12 +257,13 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
* not worth getting one just for that.
*/
read_pages(ractl);
- ractl->_index++;
- i = ractl->_index + ractl->_nr_pages - index - 1;
+ ractl->_index += min_nrpages;
+ i = ractl->_index + ractl->_nr_pages - index;
continue;
}

- folio = filemap_alloc_folio(gfp_mask, 0);
+ folio = filemap_alloc_folio(gfp_mask,
+ mapping_min_folio_order(mapping));
if (!folio)
break;

@@ -255,14 +273,15 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
if (ret == -ENOMEM)
break;
read_pages(ractl);
- ractl->_index++;
- i = ractl->_index + ractl->_nr_pages - index - 1;
+ ractl->_index += min_nrpages;
+ i = ractl->_index + ractl->_nr_pages - index;
continue;
}
- if (i == nr_to_read - lookahead_size)
+ if (i == mark)
folio_set_readahead(folio);
ractl->_workingset |= folio_test_workingset(folio);
- ractl->_nr_pages++;
+ ractl->_nr_pages += min_nrpages;
+ i += min_nrpages;
}

/*
@@ -492,13 +511,19 @@ void page_cache_ra_order(struct readahead_control *ractl,
{
struct address_space *mapping = ractl->mapping;
pgoff_t index = readahead_index(ractl);
+ unsigned int min_order = mapping_min_folio_order(mapping);
pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
pgoff_t mark = index + ra->size - ra->async_size;
unsigned int nofs;
int err = 0;
gfp_t gfp = readahead_gfp_mask(mapping);
+ unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping));

- if (!mapping_large_folio_support(mapping) || ra->size < 4)
+ /*
+ * Fallback when size < min_nrpages as each folio should be
+ * at least min_nrpages anyway.
+ */
+ if (!mapping_large_folio_support(mapping) || ra->size < min_ra_size)
goto fallback;

limit = min(limit, index + ra->size - 1);
@@ -507,11 +532,20 @@ void page_cache_ra_order(struct readahead_control *ractl,
new_order += 2;
new_order = min(mapping_max_folio_order(mapping), new_order);
new_order = min_t(unsigned int, new_order, ilog2(ra->size));
+ new_order = max(new_order, min_order);
}

/* See comment in page_cache_ra_unbounded() */
nofs = memalloc_nofs_save();
filemap_invalidate_lock_shared(mapping);
+ /*
+ * If the new_order is greater than min_order and index is
+ * already aligned to new_order, then this will be noop as index
+ * aligned to new_order should also be aligned to min_order.
+ */
+ ractl->_index = mapping_align_index(mapping, index);
+ index = readahead_index(ractl);
+
while (index <= limit) {
unsigned int order = new_order;

@@ -519,7 +553,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
if (index & ((1UL << order) - 1))
order = __ffs(index);
/* Don't allocate pages past EOF */
- while (index + (1UL << order) - 1 > limit)
+ while (order > min_order && index + (1UL << order) - 1 > limit)
order--;
err = ra_alloc_folio(ractl, index, mark, order, gfp);
if (err)
@@ -783,8 +817,15 @@ void readahead_expand(struct readahead_control *ractl,
struct file_ra_state *ra = ractl->ra;
pgoff_t new_index, new_nr_pages;
gfp_t gfp_mask = readahead_gfp_mask(mapping);
+ unsigned long min_nrpages = mapping_min_folio_nrpages(mapping);
+ unsigned int min_order = mapping_min_folio_order(mapping);

new_index = new_start / PAGE_SIZE;
+ /*
+ * Readahead code should have aligned the ractl->_index to
+ * min_nrpages before calling readahead aops.
+ */
+ VM_BUG_ON(!IS_ALIGNED(ractl->_index, min_nrpages));

/* Expand the leading edge downwards */
while (ractl->_index > new_index) {
@@ -794,9 +835,11 @@ void readahead_expand(struct readahead_control *ractl,
if (folio && !xa_is_value(folio))
return; /* Folio apparently present */

- folio = filemap_alloc_folio(gfp_mask, 0);
+ folio = filemap_alloc_folio(gfp_mask, min_order);
if (!folio)
return;
+
+ index = mapping_align_index(mapping, index);
if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
folio_put(folio);
return;
@@ -806,7 +849,7 @@ void readahead_expand(struct readahead_control *ractl,
ractl->_workingset = true;
psi_memstall_enter(&ractl->_pflags);
}
- ractl->_nr_pages++;
+ ractl->_nr_pages += min_nrpages;
ractl->_index = folio->index;
}

@@ -821,9 +864,11 @@ void readahead_expand(struct readahead_control *ractl,
if (folio && !xa_is_value(folio))
return; /* Folio apparently present */

- folio = filemap_alloc_folio(gfp_mask, 0);
+ folio = filemap_alloc_folio(gfp_mask, min_order);
if (!folio)
return;
+
+ index = mapping_align_index(mapping, index);
if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
folio_put(folio);
return;
@@ -833,10 +878,10 @@ void readahead_expand(struct readahead_control *ractl,
ractl->_workingset = true;
psi_memstall_enter(&ractl->_pflags);
}
- ractl->_nr_pages++;
+ ractl->_nr_pages += min_nrpages;
if (ra) {
- ra->size++;
- ra->async_size++;
+ ra->size += min_nrpages;
+ ra->async_size += min_nrpages;
}
}
}
--
2.44.1