[f2fs-dev] [PATCH 1/2] f2fs: fix to use bio_in_atomic() in f2fs_write_end_io()

[f2fs-dev] [PATCH 1/2] f2fs: fix to use bio_in_atomic() in f2fs_write_end_io()

[Chao Yu via Linux-f2fs-devel]

From: Chao Yu <chao@kernel.org>

f2fs_write_end_io() uses in_atomic() to decide whether a write bio which
exceeds max_atc_write_bio_size or max_atc_write_bio_entry_cnt should be
completed in sbi->wq rather than in the current context.

However, in_atomic() is not meant to be used outside core kernel code
(checkpatch.pl warns about it), as it only looks at preempt_count(): it
misses irqs-disabled and RCU read-side critical sections, and on kernels
without CONFIG_PREEMPT_COUNT it cannot see spinlock-held sections at all.
In those contexts, a large write bio will still be completed inline,
which defeats the purpose of the above knobs.

The block layer provides bio_in_atomic() for exactly this purpose, it
checks rcu_preempt_depth() and preemptible(), and conservatively returns
true if CONFIG_PREEMPT_COUNT is disabled, let's use it instead. The
difference is as below:

 Context                              | in_atomic()    | bio_in_atomic()
 -------------------------------------+----------------+----------------
 Hard IRQ / softirq                   | true           | true
 IRQs disabled, preempt count 0       | false (missed) | true
 RCU read lock (CONFIG_PREEMPTION)    | false (missed) | true
 Kernel w/o CONFIG_PREEMPT_COUNT      | IRQ ctx only   | always true

Note that on kernels without CONFIG_PREEMPT_COUNT, write bios exceeding
the thresholds will always be offloaded to sbi->wq, since there is no way
to tell whether the current context is atomic. The thresholds default to
UINT_MAX, so the default behavior is not changed.

Cc: stable@kernel.org
Fixes: 3de6b8094115 ("f2fs: Run f2fs_write_end_io() asynchronously")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/data.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 2613e8821af6..86150f7a372c 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -398,8 +398,9 @@ static void f2fs_write_end_io(struct bio *bio)
 
 	sbi = bio->bi_private;
 
-	if (in_atomic() && (bio->bi_iter.bi_size > sbi->max_atc_write_bio_size ||
-			F2FS_BIO(bio)->entry_cnt > sbi->max_atc_write_bio_entry_cnt)) {
+	if (bio_in_atomic() &&
+	    (bio->bi_iter.bi_size > sbi->max_atc_write_bio_size ||
+	     F2FS_BIO(bio)->entry_cnt > sbi->max_atc_write_bio_entry_cnt)) {
 		struct work_struct *w;
 
 		w = &container_of(bio, struct f2fs_bio, bio)->work;
-- 
2.49.0



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[f2fs-dev] [PATCH 2/2] f2fs: fix to use bio_in_atomic() in f2fs_read_end_io()

[Chao Yu via Linux-f2fs-devel]

From: Chao Yu <chao@kernel.org>

f2fs_read_end_io() uses "in_task() && !irqs_disabled()" to decide
whether it is safe to sleep, the result is passed down as @in_task to:
- f2fs_put_dic() -> f2fs_free_dic() -> f2fs_release_decomp_mem()
  -> vm_unmap_ram(), which may sleep.
- f2fs_end_read_compressed_page() -> f2fs_cache_compressed_page()
  -> f2fs_grab_cache() -> f2fs_lock_cache(), which may sleep in
  wait_on_bit_lock().

However, the check still treats below task contexts as sleepable:
- with spinlock held via spin_lock(), i.e. preempt count != 0 while
  irqs are enabled.
- with BH disabled via local_bh_disable().
- inside an RCU read lock on kernels with CONFIG_PREEMPTION.
- with any spinlock held on kernels without CONFIG_PREEMPT_COUNT.

So the "sleeping function called from invalid context" issue fixed by
commit 08a7efc5b02a ("f2fs: vm_unmap_ram() may be called from an
invalid context") can still be triggered.

The block layer provides bio_in_atomic() for exactly this purpose, it
checks rcu_preempt_depth() and preemptible(), and conservatively returns
true if CONFIG_PREEMPT_COUNT is disabled, let's use it instead. The
difference is as below:

 old: intask = in_task() && !irqs_disabled()
 new: intask = !bio_in_atomic()

 Context                              | old intask     | new intask
 -------------------------------------+----------------+----------------
 Hard IRQ / softirq                   | false          | false
 IRQs disabled                        | false          | false
 spin_lock() held, IRQs enabled       | true (wrong)   | false
 BH disabled by local_bh_disable()    | true (wrong)   | false
 RCU read lock (CONFIG_PREEMPTION)    | true (wrong)   | false
 Spinlock held, w/o PREEMPT_COUNT     | true (wrong)   | false

Note that on kernels without CONFIG_PREEMPT_COUNT, bio_in_atomic()
always returns true, so dic freeing will always be offloaded to sbi->wq,
and compressed pages will not be cached in f2fs_read_end_io(); this is
the price of correctness, since there is no way to tell whether the
current context can sleep.

Cc: stable@kernel.org
Fixes: 08a7efc5b02a ("f2fs: vm_unmap_ram() may be called from an invalid context")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/data.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 86150f7a372c..8da443fec0a6 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -271,7 +271,7 @@ static void f2fs_read_end_io(struct bio *bio)
 {
 	struct f2fs_sb_info *sbi = F2FS_F_SB(bio_first_folio_all(bio));
 	struct bio_post_read_ctx *ctx;
-	bool intask = in_task() && !irqs_disabled();
+	bool intask = !bio_in_atomic();
 
 	iostat_update_and_unbind_ctx(bio);
 	ctx = bio->bi_private;
-- 
2.49.0



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