[PATCH v4 17/39] netfs: Limit subrequest by size or number of segments

David Howells
2023-12-13T15:25:15Z

Limit a subrequest to a maximum size and/or a maximum number of contiguous
physical regions. This permits, for instance, an subreq's iterator to be
limited to the number of DMA'able segments that a large RDMA request can
handle.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
---
fs/netfs/io.c | 18 ++++++++++++++++++
include/linux/netfs.h | 1 +
include/trace/events/netfs.h | 1 +
3 files changed, 20 insertions(+)

diff --git a/fs/netfs/io.c b/fs/netfs/io.c
index d8e9cd6ce338..c80b8eed1209 100644
--- a/fs/netfs/io.c
+++ b/fs/netfs/io.c
@@ -525,6 +525,7 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
struct iov_iter *io_iter)
{
enum netfs_io_source source;
+ size_t lsize;

_enter("%llx-%llx,%llx", subreq->start, subreq->start + subreq->len, rreq->i_size);

@@ -547,13 +548,30 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
source = NETFS_INVALID_READ;
goto out;
}
+
+ if (subreq->max_nr_segs) {
+ lsize = netfs_limit_iter(io_iter, 0, subreq->len,
+ subreq->max_nr_segs);
+ if (subreq->len > lsize) {
+ subreq->len = lsize;
+ trace_netfs_sreq(subreq, netfs_sreq_trace_limited);
+ }
+ }
}

+ if (subreq->len > rreq->len)
+ pr_warn("R=%08x[%u] SREQ>RREQ %zx > %zx\n",
+ rreq->debug_id, subreq->debug_index,
+ subreq->len, rreq->len);
+
if (WARN_ON(subreq->len == 0)) {
source = NETFS_INVALID_READ;
goto out;
}

+ subreq->source = source;
+ trace_netfs_sreq(subreq, netfs_sreq_trace_prepare);
+
subreq->io_iter = *io_iter;
iov_iter_truncate(&subreq->io_iter, subreq->len);
iov_iter_advance(io_iter, subreq->len);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index a30b47e10797..406d5a3a3298 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -161,6 +161,7 @@ struct netfs_io_subrequest {
refcount_t ref;
short error; /* 0 or error that occurred */
unsigned short debug_index; /* Index in list (for debugging output) */
+ unsigned int max_nr_segs; /* 0 or max number of segments in an iterator */
enum netfs_io_source source; /* Where to read from/write to */
unsigned long flags;
#define NETFS_SREQ_COPY_TO_CACHE 0 /* Set if should copy the data to the cache */
diff --git a/include/trace/events/netfs.h b/include/trace/events/netfs.h
index beec534cbaab..fce6d0bc78e5 100644
--- a/include/trace/events/netfs.h
+++ b/include/trace/events/netfs.h
@@ -44,6 +44,7 @@
#define netfs_sreq_traces \
EM(netfs_sreq_trace_download_instead, "RDOWN") \
EM(netfs_sreq_trace_free, "FREE ") \
+ EM(netfs_sreq_trace_limited, "LIMIT") \
EM(netfs_sreq_trace_prepare, "PREP ") \
EM(netfs_sreq_trace_resubmit_short, "SHORT") \
EM(netfs_sreq_trace_submit, "SUBMT") \