[RFC PATCH 13/12] netfs: Do copy-to-cache-on-read through VM writeback

From: David Howells
Date: Wed Jul 21 2021 - 14:42:24 EST

When data is read from the server and intended to be copied to the cache,
offload the cache write to the VM writeback mechanism rather than
scheduling it immediately. This allows the downloaded data to be
superseded by local changes before it is written to the cache and means
that we no longer need to use the PG_fscache flag.

This is done by the following means:

(1) The pages just downloaded into are marked dirty in
netfs_rreq_unlock().

(2) A region of NETFS_REGION_CACHE_COPY type is added to the dirty region
list.

(3) If a region-to-be-modified overlaps the cache-copy region, the
modifications supersede the download, moving the end marker over in
netfs_merge_dirty_region().

(4) We don't really want to supersede in the middle of a region, so we may
split a pristine region so that we can supersede forwards only.

(5) We mark regions we're going to supersede with NETFS_REGION_SUPERSEDED
to prevent them getting merged whilst we're superseding them. This
flag is cleared when we're done and we may merge afterwards.

(6) Adjacent download regions are potentially mergeable.

(7) When being flushed, CACHE_COPY regions are intended only to be written
to the cache, not the server, though they may contribute data to a
cross-page chunk that has to be encrypted or compressed and sent to
the server.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---
fs/netfs/internal.h | 4 --
fs/netfs/main.c | 1
fs/netfs/read_helper.c | 126 ++--------------------------------------------------------------
fs/netfs/stats.c | 7 ---
fs/netfs/write_back.c | 3 +
fs/netfs/write_helper.c | 112 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
include/linux/netfs.h | 2 -
include/trace/events/netfs.h | 3 +
mm/filemap.c | 4 +-
9 files changed, 125 insertions(+), 137 deletions(-)


diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index 6ae1eb55093a..ee83b81e4682 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -98,6 +98,7 @@ void netfs_writeback_worker(struct work_struct *work);
void netfs_flush_region(struct netfs_i_context *ctx,
struct netfs_dirty_region *region,
enum netfs_dirty_trace why);
+void netfs_rreq_do_write_to_cache(struct netfs_read_request *rreq);

/*
* write_prep.c
@@ -121,10 +122,7 @@ extern atomic_t netfs_n_rh_read_done;
extern atomic_t netfs_n_rh_read_failed;
extern atomic_t netfs_n_rh_zero;
extern atomic_t netfs_n_rh_short_read;
-extern atomic_t netfs_n_rh_write;
extern atomic_t netfs_n_rh_write_begin;
-extern atomic_t netfs_n_rh_write_done;
-extern atomic_t netfs_n_rh_write_failed;
extern atomic_t netfs_n_rh_write_zskip;
extern atomic_t netfs_n_wh_region;
extern atomic_t netfs_n_wh_flush_group;
diff --git a/fs/netfs/main.c b/fs/netfs/main.c
index 125b570efefd..ad204dcbb5f7 100644
--- a/fs/netfs/main.c
+++ b/fs/netfs/main.c
@@ -21,6 +21,7 @@ static const char *netfs_proc_region_types[] = {
[NETFS_REGION_ORDINARY] = "ORD ",
[NETFS_REGION_DIO] = "DIOW",
[NETFS_REGION_DSYNC] = "DSYN",
+ [NETFS_REGION_CACHE_COPY] = "CCPY",
};

/*
diff --git a/fs/netfs/read_helper.c b/fs/netfs/read_helper.c
index e5c636acc756..7fa677d4c9ca 100644
--- a/fs/netfs/read_helper.c
+++ b/fs/netfs/read_helper.c
@@ -212,124 +212,6 @@ void netfs_rreq_completed(struct netfs_read_request *rreq, bool was_async)
netfs_put_read_request(rreq, was_async);
}

-/*
- * Deal with the completion of writing the data to the cache. We have to clear
- * the PG_fscache bits on the pages involved and release the caller's ref.
- *
- * May be called in softirq mode and we inherit a ref from the caller.
- */
-static void netfs_rreq_unmark_after_write(struct netfs_read_request *rreq,
- bool was_async)
-{
- struct netfs_read_subrequest *subreq;
- struct page *page;
- pgoff_t unlocked = 0;
- bool have_unlocked = false;
-
- rcu_read_lock();
-
- list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
- XA_STATE(xas, &rreq->mapping->i_pages, subreq->start / PAGE_SIZE);
-
- xas_for_each(&xas, page, (subreq->start + subreq->len - 1) / PAGE_SIZE) {
- /* We might have multiple writes from the same huge
- * page, but we mustn't unlock a page more than once.
- */
- if (have_unlocked && page->index <= unlocked)
- continue;
- unlocked = page->index;
- end_page_fscache(page);
- have_unlocked = true;
- }
- }
-
- rcu_read_unlock();
- netfs_rreq_completed(rreq, was_async);
-}
-
-static void netfs_rreq_copy_terminated(void *priv, ssize_t transferred_or_error,
- bool was_async)
-{
- struct netfs_read_subrequest *subreq = priv;
- struct netfs_read_request *rreq = subreq->rreq;
-
- if (IS_ERR_VALUE(transferred_or_error)) {
- netfs_stat(&netfs_n_rh_write_failed);
- trace_netfs_failure(rreq, subreq, transferred_or_error,
- netfs_fail_copy_to_cache);
- } else {
- netfs_stat(&netfs_n_rh_write_done);
- }
-
- trace_netfs_sreq(subreq, netfs_sreq_trace_write_term);
-
- /* If we decrement nr_wr_ops to 0, the ref belongs to us. */
- if (atomic_dec_and_test(&rreq->nr_wr_ops))
- netfs_rreq_unmark_after_write(rreq, was_async);
-
- netfs_put_subrequest(subreq, was_async);
-}
-
-/*
- * Perform any outstanding writes to the cache. We inherit a ref from the
- * caller.
- */
-static void netfs_rreq_do_write_to_cache(struct netfs_read_request *rreq)
-{
- struct netfs_cache_resources *cres = &rreq->cache_resources;
- struct netfs_read_subrequest *subreq, *next, *p;
- struct iov_iter iter;
- int ret;
-
- trace_netfs_rreq(rreq, netfs_rreq_trace_write);
-
- /* We don't want terminating writes trying to wake us up whilst we're
- * still going through the list.
- */
- atomic_inc(&rreq->nr_wr_ops);
-
- list_for_each_entry_safe(subreq, p, &rreq->subrequests, rreq_link) {
- if (!test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags)) {
- list_del_init(&subreq->rreq_link);
- netfs_put_subrequest(subreq, false);
- }
- }
-
- list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
- /* Amalgamate adjacent writes */
- while (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
- next = list_next_entry(subreq, rreq_link);
- if (next->start != subreq->start + subreq->len)
- break;
- subreq->len += next->len;
- list_del_init(&next->rreq_link);
- netfs_put_subrequest(next, false);
- }
-
- ret = cres->ops->prepare_write(cres, &subreq->start, &subreq->len,
- rreq->i_size);
- if (ret < 0) {
- trace_netfs_failure(rreq, subreq, ret, netfs_fail_prepare_write);
- trace_netfs_sreq(subreq, netfs_sreq_trace_write_skip);
- continue;
- }
-
- iov_iter_xarray(&iter, WRITE, &rreq->mapping->i_pages,
- subreq->start, subreq->len);
-
- atomic_inc(&rreq->nr_wr_ops);
- netfs_stat(&netfs_n_rh_write);
- netfs_get_read_subrequest(subreq);
- trace_netfs_sreq(subreq, netfs_sreq_trace_write);
- cres->ops->write(cres, subreq->start, &iter,
- netfs_rreq_copy_terminated, subreq);
- }
-
- /* If we decrement nr_wr_ops to 0, the usage ref belongs to us. */
- if (atomic_dec_and_test(&rreq->nr_wr_ops))
- netfs_rreq_unmark_after_write(rreq, false);
-}
-
static void netfs_rreq_write_to_cache_work(struct work_struct *work)
{
struct netfs_read_request *rreq =
@@ -390,19 +272,19 @@ static void netfs_rreq_unlock(struct netfs_read_request *rreq)
xas_for_each(&xas, page, last_page) {
unsigned int pgpos = (page->index - start_page) * PAGE_SIZE;
unsigned int pgend = pgpos + thp_size(page);
- bool pg_failed = false;
+ bool pg_failed = false, caching;

for (;;) {
if (!subreq) {
pg_failed = true;
break;
}
- if (test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags))
- set_page_fscache(page);
pg_failed |= subreq_failed;
if (pgend < iopos + subreq->len)
break;

+ caching = test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags);
+
account += subreq->len - iov_iter_count(&subreq->iter);
iopos += subreq->len;
if (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
@@ -420,6 +302,8 @@ static void netfs_rreq_unlock(struct netfs_read_request *rreq)
for (i = 0; i < thp_nr_pages(page); i++)
flush_dcache_page(page);
SetPageUptodate(page);
+ if (caching)
+ set_page_dirty(page);
}

if (!test_bit(NETFS_RREQ_DONT_UNLOCK_PAGES, &rreq->flags)) {
diff --git a/fs/netfs/stats.c b/fs/netfs/stats.c
index a02d95bba158..414c2fca6b23 100644
--- a/fs/netfs/stats.c
+++ b/fs/netfs/stats.c
@@ -22,10 +22,7 @@ atomic_t netfs_n_rh_read_done;
atomic_t netfs_n_rh_read_failed;
atomic_t netfs_n_rh_zero;
atomic_t netfs_n_rh_short_read;
-atomic_t netfs_n_rh_write;
atomic_t netfs_n_rh_write_begin;
-atomic_t netfs_n_rh_write_done;
-atomic_t netfs_n_rh_write_failed;
atomic_t netfs_n_rh_write_zskip;
atomic_t netfs_n_wh_region;
atomic_t netfs_n_wh_flush_group;
@@ -59,10 +56,6 @@ void netfs_stats_show(struct seq_file *m)
atomic_read(&netfs_n_rh_read),
atomic_read(&netfs_n_rh_read_done),
atomic_read(&netfs_n_rh_read_failed));
- seq_printf(m, "RdHelp : WR=%u ws=%u wf=%u\n",
- atomic_read(&netfs_n_rh_write),
- atomic_read(&netfs_n_rh_write_done),
- atomic_read(&netfs_n_rh_write_failed));
seq_printf(m, "WrHelp : R=%u F=%u wr=%u\n",
atomic_read(&netfs_n_wh_region),
atomic_read(&netfs_n_wh_flush_group),
diff --git a/fs/netfs/write_back.c b/fs/netfs/write_back.c
index 7363c3324602..4433c3121435 100644
--- a/fs/netfs/write_back.c
+++ b/fs/netfs/write_back.c
@@ -263,7 +263,8 @@ static void netfs_writeback(struct netfs_write_request *wreq)

if (test_bit(NETFS_WREQ_WRITE_TO_CACHE, &wreq->flags))
netfs_set_up_write_to_cache(wreq);
- ctx->ops->add_write_streams(wreq);
+ if (wreq->region->type != NETFS_REGION_CACHE_COPY)
+ ctx->ops->add_write_streams(wreq);

out:
if (atomic_dec_and_test(&wreq->outstanding))
diff --git a/fs/netfs/write_helper.c b/fs/netfs/write_helper.c
index b1fe2d4c0df6..5e50b01527fb 100644
--- a/fs/netfs/write_helper.c
+++ b/fs/netfs/write_helper.c
@@ -80,6 +80,11 @@ static void netfs_init_dirty_region(struct netfs_dirty_region *region,
INIT_LIST_HEAD(&region->flush_link);
refcount_set(&region->ref, 1);
spin_lock_init(&region->lock);
+ if (type == NETFS_REGION_CACHE_COPY) {
+ region->state = NETFS_REGION_IS_DIRTY;
+ region->dirty.end = end;
+ }
+
if (file && ctx->ops->init_dirty_region)
ctx->ops->init_dirty_region(region, file);
if (!region->group) {
@@ -160,6 +165,19 @@ static enum netfs_write_compatibility netfs_write_compatibility(
return NETFS_WRITES_INCOMPATIBLE;
}

+ /* Pending writes to the cache alone (ie. copy from a read) can be
+ * merged or superseded by a modification that will require writing to
+ * the server too.
+ */
+ if (old->type == NETFS_REGION_CACHE_COPY) {
+ if (candidate->type == NETFS_REGION_CACHE_COPY) {
+ kleave(" = COMPT [ccopy]");
+ return NETFS_WRITES_COMPATIBLE;
+ }
+ kleave(" = SUPER [ccopy]");
+ return NETFS_WRITES_SUPERSEDE;
+ }
+
if (!ctx->ops->is_write_compatible) {
if (candidate->type == NETFS_REGION_DSYNC) {
kleave(" = SUPER [dsync]");
@@ -220,8 +238,11 @@ static void netfs_queue_write(struct netfs_i_context *ctx,
if (overlaps(&candidate->bounds, &r->bounds)) {
if (overlaps(&candidate->reserved, &r->reserved) ||
netfs_write_compatibility(ctx, r, candidate) ==
- NETFS_WRITES_INCOMPATIBLE)
+ NETFS_WRITES_INCOMPATIBLE) {
+ kdebug("conflict %x with pend %x",
+ candidate->debug_id, r->debug_id);
goto add_to_pending_queue;
+ }
}
}

@@ -238,8 +259,11 @@ static void netfs_queue_write(struct netfs_i_context *ctx,
if (overlaps(&candidate->bounds, &r->bounds)) {
if (overlaps(&candidate->reserved, &r->reserved) ||
netfs_write_compatibility(ctx, r, candidate) ==
- NETFS_WRITES_INCOMPATIBLE)
+ NETFS_WRITES_INCOMPATIBLE) {
+ kdebug("conflict %x with actv %x",
+ candidate->debug_id, r->debug_id);
goto add_to_pending_queue;
+ }
}
}

@@ -451,6 +475,9 @@ static void netfs_merge_dirty_region(struct netfs_i_context *ctx,
goto discard;
}
goto scan_backwards;
+
+ case NETFS_REGION_CACHE_COPY:
+ goto scan_backwards;
}

scan_backwards:
@@ -922,3 +949,84 @@ ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
goto out;
}
EXPORT_SYMBOL(netfs_file_write_iter);
+
+/*
+ * Add a region that's just been read as a region on the dirty list to
+ * schedule a write to the cache.
+ */
+static bool netfs_copy_to_cache(struct netfs_read_request *rreq,
+ struct netfs_read_subrequest *subreq)
+{
+ struct netfs_dirty_region *candidate, *r;
+ struct netfs_i_context *ctx = netfs_i_context(rreq->inode);
+ struct list_head *p;
+ loff_t end = subreq->start + subreq->len;
+ int ret;
+
+ ret = netfs_require_flush_group(rreq->inode);
+ if (ret < 0)
+ return false;
+
+ candidate = netfs_alloc_dirty_region();
+ if (!candidate)
+ return false;
+
+ netfs_init_dirty_region(candidate, rreq->inode, NULL,
+ NETFS_REGION_CACHE_COPY, 0, subreq->start, end);
+
+ spin_lock(&ctx->lock);
+
+ /* Find a place to insert. There can't be any dirty regions
+ * overlapping with the region we're adding.
+ */
+ list_for_each(p, &ctx->dirty_regions) {
+ r = list_entry(p, struct netfs_dirty_region, dirty_link);
+ if (r->bounds.end <= candidate->bounds.start)
+ continue;
+ if (r->bounds.start >= candidate->bounds.end)
+ break;
+ }
+
+ list_add_tail(&candidate->dirty_link, p);
+ netfs_merge_dirty_region(ctx, candidate);
+
+ spin_unlock(&ctx->lock);
+ return true;
+}
+
+/*
+ * If we downloaded some data and it now needs writing to the cache, we add it
+ * to the dirty region list and let that flush it. This way it can get merged
+ * with writes.
+ *
+ * We inherit a ref from the caller.
+ */
+void netfs_rreq_do_write_to_cache(struct netfs_read_request *rreq)
+{
+ struct netfs_read_subrequest *subreq, *next, *p;
+
+ trace_netfs_rreq(rreq, netfs_rreq_trace_write);
+
+ list_for_each_entry_safe(subreq, p, &rreq->subrequests, rreq_link) {
+ if (!test_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags)) {
+ list_del_init(&subreq->rreq_link);
+ netfs_put_subrequest(subreq, false);
+ }
+ }
+
+ list_for_each_entry(subreq, &rreq->subrequests, rreq_link) {
+ /* Amalgamate adjacent writes */
+ while (!list_is_last(&subreq->rreq_link, &rreq->subrequests)) {
+ next = list_next_entry(subreq, rreq_link);
+ if (next->start != subreq->start + subreq->len)
+ break;
+ subreq->len += next->len;
+ list_del_init(&next->rreq_link);
+ netfs_put_subrequest(next, false);
+ }
+
+ netfs_copy_to_cache(rreq, subreq);
+ }
+
+ netfs_rreq_completed(rreq, false);
+}
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 43d195badb0d..527f08eb4898 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -145,7 +145,6 @@ struct netfs_read_request {
void *netfs_priv; /* Private data for the netfs */
unsigned int debug_id;
atomic_t nr_rd_ops; /* Number of read ops in progress */
- atomic_t nr_wr_ops; /* Number of write ops in progress */
size_t submitted; /* Amount submitted for I/O so far */
size_t len; /* Length of the request */
short error; /* 0 or error that occurred */
@@ -218,6 +217,7 @@ enum netfs_region_type {
NETFS_REGION_ORDINARY, /* Ordinary write */
NETFS_REGION_DIO, /* Direct I/O write */
NETFS_REGION_DSYNC, /* O_DSYNC/RWF_DSYNC write */
+ NETFS_REGION_CACHE_COPY, /* Data to be written to cache only */
} __attribute__((mode(byte)));

/*
diff --git a/include/trace/events/netfs.h b/include/trace/events/netfs.h
index aa002725b209..136cc42263f9 100644
--- a/include/trace/events/netfs.h
+++ b/include/trace/events/netfs.h
@@ -156,7 +156,8 @@ enum netfs_write_stream_trace {
#define netfs_region_types \
EM(NETFS_REGION_ORDINARY, "ORD") \
EM(NETFS_REGION_DIO, "DIO") \
- E_(NETFS_REGION_DSYNC, "DSY")
+ EM(NETFS_REGION_DSYNC, "DSY") \
+ E_(NETFS_REGION_CACHE_COPY, "CCP")

#define netfs_region_states \
EM(NETFS_REGION_IS_PENDING, "pend") \
diff --git a/mm/filemap.c b/mm/filemap.c
index d1458ecf2f51..442cd767a047 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1545,8 +1545,10 @@ void end_page_writeback(struct page *page)
* reused before the wake_up_page().
*/
get_page(page);
- if (!test_clear_page_writeback(page))
+ if (!test_clear_page_writeback(page)) {
+ pr_err("Page %lx doesn't have wb set\n", page->index);
BUG();
+ }

smp_mb__after_atomic();
wake_up_page(page, PG_writeback);