Re: [PATCH v2 2/4] brd: extend the rcu regions to cover read and write
From: Christoph Hellwig <hch@infradead.org>
Date: 2022-09-23 15:53:13
Also in:
dm-devel
On Tue, Sep 20, 2022 at 01:56:25PM -0400, Mikulas Patocka wrote:
quoted hunk ↗ jump to hunk
This patch extends the rcu regions, so that lookup followed by a read or write of a page is done inside rcu read lock. This si be needed for the following patch that enables discard. Note that we also replace "BUG_ON(!page);" with "if (page) ..." in copy_to_brd - the page may be NULL if write races with discard. In this situation, the result is undefined, so we can actually skip the write operation at all. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> --- drivers/block/brd.c | 59 +++++++++++++++++++++++----------------------------- 1 file changed, 27 insertions(+), 32 deletions(-) Index: linux-2.6/drivers/block/brd.c ===================================================================--- linux-2.6.orig/drivers/block/brd.c +++ linux-2.6/drivers/block/brd.c@@ -50,31 +50,12 @@ struct brd_device { /* * Look up and return a brd's page for a given sector. + * This must be called with the rcu lock held. */ static struct page *brd_lookup_page(struct brd_device *brd, sector_t sector) { + pgoff_t idx = sector >> PAGE_SECTORS_SHIFT; /* sector to page index */ + return radix_tree_lookup(&brd->brd_pages, idx); }
This is still missing the rcu_read_lock_held() assertation if you want to keep it as separate function.
+ rcu_read_lock();
+ page = brd_lookup_page(brd, sector);
+ if (page) {
+ dst = kmap_atomic(page);
+ memcpy(dst + offset, src, copy);
+ kunmap_atomic(dst);
+ }
+ rcu_read_unlock();How is the null check going to work here? Simply not copying data is no exactly the expected result. This is why I think we need the higher level rework I suggested last time where we have a helper that always gives you page (or maybe an error) by moving the insert so that it also does the actual final lookup.