[PATCH 7/7] nfs: don't reuse partially completed requests in nfs_lock_and_join_requests

Christoph Hellwig
2024-07-01T05:27:30Z

When NFS requests are split into sub-requests, nfs_inode_remove_request
calls nfs_page_group_sync_on_bit to set PG_REMOVE on this sub-request and
only completes the head requests once PG_REMOVE is set on all requests.
This means that when nfs_lock_and_join_requests sees a PG_REMOVE bit, I/O
on the request is in progress and has partially completed. If such a
request is returned to nfs_try_to_update_request, it could be extended
with the newly dirtied region and I/O for the combined range will be
re-scheduled, leading to extra I/O.

Change the logic to instead restart the search for a request when any
PG_REMOVE bit is set, as the completion handler will remove the request
as soon as it can take the page group lock. This not only avoid
extending the I/O but also does the right thing for the callers that
want to cancel or flush the request.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
fs/nfs/write.c | 49 ++++++++++++++++++++-----------------------------
1 file changed, 20 insertions(+), 29 deletions(-)

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 2c089444303982..4dffdc5aadb2e2 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -144,31 +144,6 @@ static void nfs_io_completion_put(struct nfs_io_completion *ioc)
kref_put(&ioc->refcount, nfs_io_completion_release);
}

-static void
-nfs_page_set_inode_ref(struct nfs_page *req, struct inode *inode)
-{
- if (!test_and_set_bit(PG_INODE_REF, &req->wb_flags)) {
- kref_get(&req->wb_kref);
- atomic_long_inc(&NFS_I(inode)->nrequests);
- }
-}
-
-static int
-nfs_cancel_remove_inode(struct nfs_page *req, struct inode *inode)
-{
- int ret;
-
- if (!test_bit(PG_REMOVE, &req->wb_flags))
- return 0;
- ret = nfs_page_group_lock(req);
- if (ret)
- return ret;
- if (test_and_clear_bit(PG_REMOVE, &req->wb_flags))
- nfs_page_set_inode_ref(req, inode);
- nfs_page_group_unlock(req);
- return 0;
-}
-
/**
* nfs_folio_find_head_request - find head request associated with a folio
* @folio: pointer to folio
@@ -564,6 +539,7 @@ static struct nfs_page *nfs_lock_and_join_requests(struct folio *folio)
struct inode *inode = folio->mapping->host;
struct nfs_page *head, *subreq;
struct nfs_commit_info cinfo;
+ bool removed;
int ret;

/*
@@ -588,18 +564,18 @@ static struct nfs_page *nfs_lock_and_join_requests(struct folio *folio)
goto retry;
}

- ret = nfs_cancel_remove_inode(head, inode);
- if (ret < 0)
- goto out_unlock;
-
ret = nfs_page_group_lock(head);
if (ret < 0)
goto out_unlock;

+ removed = test_bit(PG_REMOVE, &head->wb_flags);
+
/* lock each request in the page group */
for (subreq = head->wb_this_page;
subreq != head;
subreq = subreq->wb_this_page) {
+ if (test_bit(PG_REMOVE, &subreq->wb_flags))
+ removed = true;
ret = nfs_page_group_lock_subreq(head, subreq);
if (ret < 0)
goto out_unlock;
@@ -607,6 +583,21 @@ static struct nfs_page *nfs_lock_and_join_requests(struct folio *folio)

nfs_page_group_unlock(head);

+ /*
+ * If PG_REMOVE is set on any request, I/O on that request has
+ * completed, but some requests were still under I/O at the time
+ * we locked the head request.
+ *
+ * In that case the above wait for all requests means that all I/O
+ * has now finished, and we can restart from a clean slate. Let the
+ * old requests go away and start from scratch instead.
+ */
+ if (removed) {
+ nfs_unroll_locks(head, head);
+ nfs_unlock_and_release_request(head);
+ goto retry;
+ }
+
nfs_init_cinfo_from_inode(&cinfo, inode);
nfs_join_page_group(head, &cinfo, inode);
return head;
--
2.43.0