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Thu, 04 Mar 2021 23:28:47 -0800 (PST) Subject: Re: [PATCH] md: bcache: fix error return code of cached_dev_cache_miss() To: Coly Li Cc: linux-bcache@vger.kernel.org, linux-kernel@vger.kernel.org, kent.overstreet@gmail.com References: <20210305024609.4880-1-baijiaju1990@gmail.com> From: Jia-Ju Bai Message-ID: <3324a8b6-51a8-2a06-9d3d-9896ace8ef66@gmail.com> Date: Fri, 5 Mar 2021 15:28:34 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-bcache@vger.kernel.org Hi Coly, Thanks a lot for your detailed explanation :) Best wishes, Jia-Ju Bai On 2021/3/5 12:05, Coly Li wrote: > On 3/5/21 10:46 AM, Jia-Ju Bai wrote: >> When bch_bio_alloc_pages() fails, no error return code of >> cached_dev_cache_miss() is assigned. >> To fix this bug, ret is assigned with -ENOMEN as error return code. >> >> Reported-by: TOTE Robot >> Signed-off-by: Jia-Ju Bai >> --- >> drivers/md/bcache/request.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c >> index 29c231758293..9ecaf26c8d60 100644 >> --- a/drivers/md/bcache/request.c >> +++ b/drivers/md/bcache/request.c >> @@ -930,8 +930,10 @@ static int cached_dev_cache_miss(struct btree *b, struct search *s, >> cache_bio->bi_private = &s->cl; >> >> bch_bio_map(cache_bio, NULL); >> - if (bch_bio_alloc_pages(cache_bio, __GFP_NOWARN|GFP_NOIO)) >> + if (bch_bio_alloc_pages(cache_bio, __GFP_NOWARN|GFP_NOIO)) { >> + ret = -ENOMEM; >> goto out_put; >> + } >> >> if (reada) >> bch_mark_cache_readahead(s->iop.c, s->d); >> > Thanks for looking at bcache :-) > > Without the above change, -EINTR will be returned. -EINTR is special in > bache's btree iteration code. See bcache_btree_root() from bcache.h, > > 347 #define bcache_btree_root(fn, c, op, ...) \ > 348 ({ \ > 349 int _r = -EINTR; \ > 350 do { \ > 351 struct btree *_b = (c)->root; \ > 352 bool _w = insert_lock(op, _b); \ > 353 rw_lock(_w, _b, _b->level); \ > 354 if (_b == (c)->root && \ > 355 _w == insert_lock(op, _b)) { \ > 356 _r = bch_btree_ ## fn(_b, op, ##__VA_ARGS__); \ > 357 } \ > 358 rw_unlock(_w, _b); \ > 359 bch_cannibalize_unlock(c); \ > 360 if (_r == -EINTR) \ > 361 \ > 362 } while (_r == -EINTR); \ > 363 \ > 364 finish_wait(&(c)->btree_cache_wait, &(op)->wait); \ > 365 _r; \ > 366 }) > > cached_dev_cache_miss() is called by the following code path, > > cache_lookup() ==> bch_btree_map_keys() ==> bcache_btree_root() ==> > bch_btree_map_keys_recurse() ==> cache_lookup_fn() > > Therefore the return value of cached_dev_cache_miss() will be returned > from where s->d->cache_miss() is called from cache_lookup_fn(). And in > macro bcache_btree_root() this return value will be checked. If the > return value is -EINTR, then the whole iteration will be re-do again. > > Returning -ENOMEM works but if the memory allocation failed, there is no > chance to re-do the cache lookup again from bcache_btree_root(). When > system memory is in heavy usage, we want the lookup to try more times > (because GFP_NOIO is set), which is much better then returning -EIO > immediately to caller. > > Therefore NOT setting ret to -ENOMEM in the patching location should be > an on-purpose coding, IMHO. > > Thanks. > > Coly Li >