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brd: avoid extra xarray lookups on first write

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The xarray can return the previous entry at a location.  Use this
fact to simplify the brd code when there is no existing page at
a location.  This also slighly improves the handling of racy
discards as we now always have a page under RCU protection by the
time we are ready to copy the data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250507060700.3929430-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Christoph Hellwig 2025-05-07 08:06:35 +02:00 committed by Jens Axboe
parent 1e332795d0
commit bbcacab2e8
1 changed files with 33 additions and 43 deletions
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76
drivers/block/brd.c
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@ -54,32 +54,33 @@ static struct page *brd_lookup_page(struct brd_device *brd, sector_t sector)
/*
* Insert a new page for a given sector, if one does not already exist.
*/
static int brd_insert_page(struct brd_device *brd, sector_t sector, gfp_t gfp)
static struct page *brd_insert_page(struct brd_device *brd, sector_t sector,
blk_opf_t opf)
__releases(rcu)
__acquires(rcu)
{
pgoff_t idx = sector >> PAGE_SECTORS_SHIFT;
struct page *page;
int ret = 0;
page = brd_lookup_page(brd, sector);
if (page)
return 0;
gfp_t gfp = (opf & REQ_NOWAIT) ? GFP_NOWAIT : GFP_NOIO;
struct page *page, *ret;
rcu_read_unlock();
page = alloc_page(gfp | __GFP_ZERO | __GFP_HIGHMEM);
rcu_read_lock();
if (!page)
return -ENOMEM;
return ERR_PTR(-ENOMEM);
xa_lock(&brd->brd_pages);
ret = __xa_insert(&brd->brd_pages, idx, page, gfp);
if (!ret)
brd->brd_nr_pages++;
xa_unlock(&brd->brd_pages);
if (ret < 0) {
ret = __xa_cmpxchg(&brd->brd_pages, sector >> PAGE_SECTORS_SHIFT, NULL,
page, gfp);
if (ret) {
xa_unlock(&brd->brd_pages);
__free_page(page);
if (ret == -EBUSY)
ret = 0;
if (xa_is_err(ret))
return ERR_PTR(xa_err(ret));
return ret;
}
return ret;
brd->brd_nr_pages++;
xa_unlock(&brd->brd_pages);
return page;
}
/*
@ -114,36 +115,17 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)
bv.bv_len = min_t(u32, bv.bv_len, PAGE_SIZE - offset);
if (op_is_write(opf)) {
int err;
/*
* Must use NOIO because we don't want to recurse back into the
* block or filesystem layers from page reclaim.
*/
err = brd_insert_page(brd, sector,
(opf & REQ_NOWAIT) ? GFP_NOWAIT : GFP_NOIO);
if (err) {
if (err == -ENOMEM && (opf & REQ_NOWAIT))
bio_wouldblock_error(bio);
else
bio_io_error(bio);
return false;
}
}
rcu_read_lock();
page = brd_lookup_page(brd, sector);
if (!page && op_is_write(opf)) {
page = brd_insert_page(brd, sector, opf);
if (IS_ERR(page))
goto out_error;
}
kaddr = bvec_kmap_local(&bv);
if (op_is_write(opf)) {
/*
* Page can be removed by concurrent discard, it's fine to skip
* the write and user will read zero data if page does not
* exist.
*/
if (page)
memcpy_to_page(page, offset, kaddr, bv.bv_len);
memcpy_to_page(page, offset, kaddr, bv.bv_len);
} else {
if (page)
memcpy_from_page(kaddr, page, offset, bv.bv_len);
@ -155,6 +137,14 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)
bio_advance_iter_single(bio, &bio->bi_iter, bv.bv_len);
return true;
out_error:
rcu_read_unlock();
if (PTR_ERR(page) == -ENOMEM && (opf & REQ_NOWAIT))
bio_wouldblock_error(bio);
else
bio_io_error(bio);
return false;
}
static void brd_free_one_page(struct rcu_head *head)