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brd: use page reference to protect page lifetime #62

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brd: use page reference to protect page lifetime #62

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75 changes: 48 additions & 27 deletions drivers/block/brd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -44,45 +44,74 @@ struct brd_device {
};

/*
* Look up and return a brd's page for a given sector.
* Look up and return a brd's page with reference grabbed for a given sector.
*/
static struct page *brd_lookup_page(struct brd_device *brd, sector_t sector)
{
return xa_load(&brd->brd_pages, sector >> PAGE_SECTORS_SHIFT);
struct page *page;
XA_STATE(xas, &brd->brd_pages, sector >> PAGE_SECTORS_SHIFT);

rcu_read_lock();
repeat:
page = xas_load(&xas);
if (xas_retry(&xas, page)) {
xas_reset(&xas);
goto repeat;
}

if (!page)
goto out;

if (!get_page_unless_zero(page)) {
xas_reset(&xas);
goto repeat;
}

if (unlikely(page != xas_reload(&xas))) {
put_page(page);
xas_reset(&xas);
goto repeat;
}
out:
rcu_read_unlock();

return page;
}

/*
* Insert a new page for a given sector, if one does not already exist.
* The returned page will grab reference.
*/
static struct page *brd_insert_page(struct brd_device *brd, sector_t sector,
blk_opf_t opf)
__releases(rcu)
__acquires(rcu)
{
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);
if (!page) {
rcu_read_lock();
if (!page)
return ERR_PTR(-ENOMEM);
}

xa_lock(&brd->brd_pages);
ret = __xa_cmpxchg(&brd->brd_pages, sector >> PAGE_SECTORS_SHIFT, NULL,
page, gfp);
rcu_read_lock();
if (ret) {
if (!ret) {
brd->brd_nr_pages++;
get_page(page);
xa_unlock(&brd->brd_pages);
return page;
}

if (!xa_is_err(ret)) {
get_page(ret);
xa_unlock(&brd->brd_pages);
__free_page(page);
if (xa_is_err(ret))
return ERR_PTR(xa_err(ret));
put_page(page);
return ret;
}
brd->brd_nr_pages++;

xa_unlock(&brd->brd_pages);
return page;
put_page(page);
return ERR_PTR(xa_err(ret));
}

/*
Expand All @@ -95,7 +124,7 @@ static void brd_free_pages(struct brd_device *brd)
pgoff_t idx;

xa_for_each(&brd->brd_pages, idx, page) {
__free_page(page);
put_page(page);
cond_resched();
}

Expand All @@ -117,7 +146,6 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)

bv.bv_len = min_t(u32, bv.bv_len, PAGE_SIZE - offset);

rcu_read_lock();
page = brd_lookup_page(brd, sector);
if (!page && op_is_write(opf)) {
page = brd_insert_page(brd, sector, opf);
Expand All @@ -135,27 +163,20 @@ static bool brd_rw_bvec(struct brd_device *brd, struct bio *bio)
memset(kaddr, 0, bv.bv_len);
}
kunmap_local(kaddr);
rcu_read_unlock();

bio_advance_iter_single(bio, &bio->bi_iter, bv.bv_len);
if (page)
put_page(page);
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)
{
struct page *page = container_of(head, struct page, rcu_head);

__free_page(page);
}

static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
{
sector_t aligned_sector = round_up(sector, PAGE_SECTORS);
Expand All @@ -170,7 +191,7 @@ static void brd_do_discard(struct brd_device *brd, sector_t sector, u32 size)
while (aligned_sector < aligned_end && aligned_sector < rd_size * 2) {
page = __xa_erase(&brd->brd_pages, aligned_sector >> PAGE_SECTORS_SHIFT);
if (page) {
call_rcu(&page->rcu_head, brd_free_one_page);
put_page(page);
brd->brd_nr_pages--;
}
aligned_sector += PAGE_SECTORS;
Expand Down
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