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Wed, 7 Jan 2026 11:50:06 +0800 (CST) X-Sender: zhangdongdong925@sina.com X-Auth-ID: zhangdongdong925@sina.com X-SMAIL-MID: 6079074456895 X-SMAIL-UIID: B76B5308995B42AD8E20F2E5FD0EC548-20260107-115006-1 Message-ID: <40e38fa7-725b-407a-917a-59c5a76dedcb@sina.com> Date: Wed, 7 Jan 2026 11:50:04 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2 1/7] zram: introduce compressed data writeback To: Sergey Senozhatsky , Andrew Morton , Richard Chang , Minchan Kim Cc: Brian Geffon , David Stevens , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, Minchan Kim References: <20251201094754.4149975-1-senozhatsky@chromium.org> <20251201094754.4149975-2-senozhatsky@chromium.org> Content-Language: en-US From: zhangdongdong In-Reply-To: <20251201094754.4149975-2-senozhatsky@chromium.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: C836C80008 X-Stat-Signature: s9cwxo4aesm5f8iu19g67mdnkos5nfkj X-HE-Tag: 1767757811-340377 X-HE-Meta: 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 13wZXzEM j0kCVnISwPQP7QnkGBhmAhprjKEwmEEUrh2Da8lI8TMbqqZpr/YcOD4XFJN2rAeqzmiawiWyBenHkWZZifSP71V+w4WURCx3K9AYdP7goYin2Pmz+BetVuVeAiNbjzRfoZKdjgdBUZSB5e6JPxz55eU68ics3t7ryY+e8XIl3+by4S3nDuDuxSir7fQnW0ABuifrtSE8jPX7Aw9GCfNfmJtk7gFbv0BIbajYwTxGBWYJ9crNVXad0PoLgkxGDqFdlsn/HvIoWGUixvfJWe+a1X30UkVl7kcq8nkiBoOKp3j07/8v2KAkI1YE4qo2c3zz22Iv7DX7raSO8OAKmuBrx2hOIRDXQQoULIedjo1hzNq8BkyXUb/WiQ6Or9tLXdfjoVP7WWVO0HAAGrov6zG/WmLGsdg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 12/1/25 17:47, Sergey Senozhatsky wrote: > From: Richard Chang > > zram stores all written back slots raw, which implies that > during writeback zram first has to decompress slots (except > for ZRAM_HUGE slots, which are raw already). The problem > with this approach is that not every written back page gets > read back (either via read() or via page-fault), which means > that zram basically wastes CPU cycles and battery decompressing > such slots. This changes with introduction of decompression > on demand, in other words decompression on read()/page-fault. > > One caveat of decompression on demand is that async read > is completed in IRQ context, while zram decompression is > sleepable. To workaround this, read-back decompression > is offloaded to a preemptible context - system high-prio > work-queue. > > At this point compressed writeback is still disabled, > a follow up patch will introduce a new device attribute > which will make it possible to toggle compressed writeback > per-device. > > [senozhatsky: rewrote original implementation] > Signed-off-by: Richard Chang > Co-developed-by: Sergey Senozhatsky > Suggested-by: Minchan Kim > Suggested-by: Brian Geffon > --- > drivers/block/zram/zram_drv.c | 279 +++++++++++++++++++++++++++------- > drivers/block/zram/zram_drv.h | 1 + > 2 files changed, 227 insertions(+), 53 deletions(-) > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c > index 5759823d6314..6263d300312e 100644 > --- a/drivers/block/zram/zram_drv.c > +++ b/drivers/block/zram/zram_drv.c > @@ -57,9 +57,6 @@ static size_t huge_class_size; > static const struct block_device_operations zram_devops; > > static void zram_free_page(struct zram *zram, size_t index); > -static int zram_read_from_zspool(struct zram *zram, struct page *page, > - u32 index); > - > #define slot_dep_map(zram, index) (&(zram)->table[(index)].dep_map) > > static void zram_slot_lock_init(struct zram *zram, u32 index) > @@ -502,6 +499,10 @@ static ssize_t idle_store(struct device *dev, > #ifdef CONFIG_ZRAM_WRITEBACK > #define INVALID_BDEV_BLOCK (~0UL) > > +static int read_from_zspool_raw(struct zram *zram, struct page *page, > + u32 index); > +static int read_from_zspool(struct zram *zram, struct page *page, u32 index); > + > struct zram_wb_ctl { > /* idle list is accessed only by the writeback task, no concurency */ > struct list_head idle_reqs; > @@ -522,6 +523,22 @@ struct zram_wb_req { > struct list_head entry; > }; > > +struct zram_rb_req { > + struct work_struct work; > + struct zram *zram; > + struct page *page; > + /* The read bio for backing device */ > + struct bio *bio; > + unsigned long blk_idx; > + union { > + /* The original bio to complete (async read) */ > + struct bio *parent; > + /* error status (sync read) */ > + int error; > + }; > + u32 index; > +}; > + > static ssize_t writeback_limit_enable_store(struct device *dev, > struct device_attribute *attr, > const char *buf, size_t len) > @@ -780,18 +797,6 @@ static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx) > atomic64_dec(&zram->stats.bd_count); > } > > -static void read_from_bdev_async(struct zram *zram, struct page *page, > - unsigned long entry, struct bio *parent) > -{ > - struct bio *bio; > - > - bio = bio_alloc(zram->bdev, 1, parent->bi_opf, GFP_NOIO); > - bio->bi_iter.bi_sector = entry * (PAGE_SIZE >> 9); > - __bio_add_page(bio, page, PAGE_SIZE, 0); > - bio_chain(bio, parent); > - submit_bio(bio); > -} > - > static void release_wb_req(struct zram_wb_req *req) > { > __free_page(req->page); > @@ -886,8 +891,9 @@ static void zram_account_writeback_submit(struct zram *zram) > > static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req) > { > - u32 index = req->pps->index; > - int err; > + u32 size, index = req->pps->index; > + int err, prio; > + bool huge; > > err = blk_status_to_errno(req->bio.bi_status); > if (err) { > @@ -914,9 +920,27 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req) > goto out; > } > > + if (zram->wb_compressed) { > + /* > + * ZRAM_WB slots get freed, we need to preserve data required > + * for read decompression. > + */ > + size = zram_get_obj_size(zram, index); > + prio = zram_get_priority(zram, index); > + huge = zram_test_flag(zram, index, ZRAM_HUGE); > + } > + > zram_free_page(zram, index); > zram_set_flag(zram, index, ZRAM_WB); > zram_set_handle(zram, index, req->blk_idx); > + > + if (zram->wb_compressed) { > + if (huge) > + zram_set_flag(zram, index, ZRAM_HUGE); > + zram_set_obj_size(zram, index, size); > + zram_set_priority(zram, index, prio); > + } > + > atomic64_inc(&zram->stats.pages_stored); > > out: > @@ -1050,7 +1074,11 @@ static int zram_writeback_slots(struct zram *zram, > */ > if (!zram_test_flag(zram, index, ZRAM_PP_SLOT)) > goto next; > - if (zram_read_from_zspool(zram, req->page, index)) > + if (zram->wb_compressed) > + err = read_from_zspool_raw(zram, req->page, index); > + else > + err = read_from_zspool(zram, req->page, index); > + if (err) > goto next; > zram_slot_unlock(zram, index); > > @@ -1313,24 +1341,140 @@ static ssize_t writeback_store(struct device *dev, > return ret; > } > > -struct zram_work { > - struct work_struct work; > - struct zram *zram; > - unsigned long entry; > - struct page *page; > - int error; > -}; > +static int decompress_bdev_page(struct zram *zram, struct page *page, u32 index) > +{ > + struct zcomp_strm *zstrm; > + unsigned int size; > + int ret, prio; > + void *src; > + > + zram_slot_lock(zram, index); > + /* Since slot was unlocked we need to make sure it's still ZRAM_WB */ > + if (!zram_test_flag(zram, index, ZRAM_WB)) { > + zram_slot_unlock(zram, index); > + /* We read some stale data, zero it out */ > + memset_page(page, 0, 0, PAGE_SIZE); > + return -EIO; > + } > + > + if (zram_test_flag(zram, index, ZRAM_HUGE)) { > + zram_slot_unlock(zram, index); > + return 0; > + } > + > + size = zram_get_obj_size(zram, index); > + prio = zram_get_priority(zram, index); > > -static void zram_sync_read(struct work_struct *work) > + zstrm = zcomp_stream_get(zram->comps[prio]); > + src = kmap_local_page(page); > + ret = zcomp_decompress(zram->comps[prio], zstrm, src, size, > + zstrm->local_copy); > + if (!ret) > + copy_page(src, zstrm->local_copy); > + kunmap_local(src); > + zcomp_stream_put(zstrm); > + zram_slot_unlock(zram, index); > + > + return ret; > +} > + > +static void zram_deferred_decompress(struct work_struct *w) > { > - struct zram_work *zw = container_of(work, struct zram_work, work); > + struct zram_rb_req *req = container_of(w, struct zram_rb_req, work); > + struct page *page = bio_first_page_all(req->bio); > + struct zram *zram = req->zram; > + u32 index = req->index; > + int ret; > + > + ret = decompress_bdev_page(zram, page, index); > + if (ret) > + req->parent->bi_status = BLK_STS_IOERR; > + > + /* Decrement parent's ->remaining */ > + bio_endio(req->parent); > + bio_put(req->bio); > + kfree(req); > +} > + > +static void zram_async_read_endio(struct bio *bio) > +{ > + struct zram_rb_req *req = bio->bi_private; > + struct zram *zram = req->zram; > + > + if (bio->bi_status) { > + req->parent->bi_status = bio->bi_status; > + bio_endio(req->parent); > + bio_put(bio); > + kfree(req); > + return; > + } > + > + /* > + * NOTE: zram_async_read_endio() is not exactly right place for this. > + * Ideally, we need to do it after ZRAM_WB check, but this requires > + * us to use wq path even on systems that don't enable compressed > + * writeback, because we cannot take slot-lock in the current context. > + * > + * Keep the existing behavior for now. > + */ > + if (zram->wb_compressed == false) { > + /* No decompression needed, complete the parent IO */ > + bio_endio(req->parent); > + bio_put(bio); > + kfree(req); > + return; > + } > + > + /* > + * zram decompression is sleepable, so we need to deffer it to > + * a preemptible context. > + */ > + INIT_WORK(&req->work, zram_deferred_decompress); > + queue_work(system_highpri_wq, &req->work); > +} > + > +static void read_from_bdev_async(struct zram *zram, struct page *page, > + u32 index, unsigned long blk_idx, > + struct bio *parent) > +{ > + struct zram_rb_req *req; > + struct bio *bio; > + > + req = kmalloc(sizeof(*req), GFP_NOIO); > + if (!req) > + return; > + > + bio = bio_alloc(zram->bdev, 1, parent->bi_opf, GFP_NOIO); > + if (!bio) { > + kfree(req); > + return; > + } > + > + req->zram = zram; > + req->index = index; > + req->blk_idx = blk_idx; > + req->bio = bio; > + req->parent = parent; > + > + bio->bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9); > + bio->bi_private = req; > + bio->bi_end_io = zram_async_read_endio; > + > + __bio_add_page(bio, page, PAGE_SIZE, 0); > + bio_inc_remaining(parent); > + submit_bio(bio); > +} > + > +static void zram_sync_read(struct work_struct *w) > +{ > + struct zram_rb_req *req = container_of(w, struct zram_rb_req, work); > struct bio_vec bv; > struct bio bio; > > - bio_init(&bio, zw->zram->bdev, &bv, 1, REQ_OP_READ); > - bio.bi_iter.bi_sector = zw->entry * (PAGE_SIZE >> 9); > - __bio_add_page(&bio, zw->page, PAGE_SIZE, 0); > - zw->error = submit_bio_wait(&bio); > + bio_init(&bio, req->zram->bdev, &bv, 1, REQ_OP_READ); > + bio.bi_iter.bi_sector = req->blk_idx * (PAGE_SIZE >> 9); > + __bio_add_page(&bio, req->page, PAGE_SIZE, 0); > + req->error = submit_bio_wait(&bio); > } > > /* > @@ -1338,39 +1482,42 @@ static void zram_sync_read(struct work_struct *work) > * chained IO with parent IO in same context, it's a deadlock. To avoid that, > * use a worker thread context. > */ > -static int read_from_bdev_sync(struct zram *zram, struct page *page, > - unsigned long entry) > +static int read_from_bdev_sync(struct zram *zram, struct page *page, u32 index, > + unsigned long blk_idx) > { > - struct zram_work work; > + struct zram_rb_req req; > > - work.page = page; > - work.zram = zram; > - work.entry = entry; > + req.page = page; > + req.zram = zram; > + req.blk_idx = blk_idx; > > - INIT_WORK_ONSTACK(&work.work, zram_sync_read); > - queue_work(system_dfl_wq, &work.work); > - flush_work(&work.work); > - destroy_work_on_stack(&work.work); > + INIT_WORK_ONSTACK(&req.work, zram_sync_read); > + queue_work(system_dfl_wq, &req.work); > + flush_work(&req.work); > + destroy_work_on_stack(&req.work); Hi Sergey, Thanks for the work on decompression-on-demand. One concern I’d like to raise is the use of a workqueue for readback decompression. In our measurements, deferring decompression to a worker introduces non-trivial scheduling overhead, and under memory pressure the added latency can be noticeable (tens of milliseconds in some cases). This makes the swap-in read path more sensitive to scheduler behavior. It may be worth considering whether the decompression can be placed in a context that avoids this extra scheduling hop, for example by moving the decompression closer to the swap layer. Thanks, dongdong > > - return work.error; > + if (req.error || zram->wb_compressed == false) > + return req.error; > + > + return decompress_bdev_page(zram, page, index); > } > > -static int read_from_bdev(struct zram *zram, struct page *page, > - unsigned long entry, struct bio *parent) > +static int read_from_bdev(struct zram *zram, struct page *page, u32 index, > + unsigned long blk_idx, struct bio *parent) > { > atomic64_inc(&zram->stats.bd_reads); > if (!parent) { > if (WARN_ON_ONCE(!IS_ENABLED(ZRAM_PARTIAL_IO))) > return -EIO; > - return read_from_bdev_sync(zram, page, entry); > + return read_from_bdev_sync(zram, page, index, blk_idx); > } > - read_from_bdev_async(zram, page, entry, parent); > + read_from_bdev_async(zram, page, index, blk_idx, parent); > return 0; > } > #else > static inline void reset_bdev(struct zram *zram) {}; > -static int read_from_bdev(struct zram *zram, struct page *page, > - unsigned long entry, struct bio *parent) > +static int read_from_bdev(struct zram *zram, struct page *page, u32 index, > + unsigned long blk_idx, struct bio *parent) > { > return -EIO; > } > @@ -1977,12 +2124,37 @@ static int read_compressed_page(struct zram *zram, struct page *page, u32 index) > return ret; > } > > +#if defined CONFIG_ZRAM_WRITEBACK > +static int read_from_zspool_raw(struct zram *zram, struct page *page, u32 index) > +{ > + struct zcomp_strm *zstrm; > + unsigned long handle; > + unsigned int size; > + void *src; > + > + handle = zram_get_handle(zram, index); > + size = zram_get_obj_size(zram, index); > + > + /* > + * We need to get stream just for ->local_copy buffer, in > + * case if object spans two physical pages. No decompression > + * takes place here, as we read raw compressed data. > + */ > + zstrm = zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP]); > + src = zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy); > + memcpy_to_page(page, 0, src, size); > + zs_obj_read_end(zram->mem_pool, handle, src); > + zcomp_stream_put(zstrm); > + > + return 0; > +} > +#endif > + > /* > * Reads (decompresses if needed) a page from zspool (zsmalloc). > * Corresponding ZRAM slot should be locked. > */ > -static int zram_read_from_zspool(struct zram *zram, struct page *page, > - u32 index) > +static int read_from_zspool(struct zram *zram, struct page *page, u32 index) > { > if (zram_test_flag(zram, index, ZRAM_SAME) || > !zram_get_handle(zram, index)) > @@ -2002,7 +2174,7 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index, > zram_slot_lock(zram, index); > if (!zram_test_flag(zram, index, ZRAM_WB)) { > /* Slot should be locked through out the function call */ > - ret = zram_read_from_zspool(zram, page, index); > + ret = read_from_zspool(zram, page, index); > zram_slot_unlock(zram, index); > } else { > unsigned long blk_idx = zram_get_handle(zram, index); > @@ -2012,7 +2184,7 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index, > * device. > */ > zram_slot_unlock(zram, index); > - ret = read_from_bdev(zram, page, blk_idx, parent); > + ret = read_from_bdev(zram, page, index, blk_idx, parent); > } > > /* Should NEVER happen. Return bio error if it does. */ > @@ -2273,7 +2445,7 @@ static int recompress_slot(struct zram *zram, u32 index, struct page *page, > if (comp_len_old < threshold) > return 0; > > - ret = zram_read_from_zspool(zram, page, index); > + ret = read_from_zspool(zram, page, index); > if (ret) > return ret; > > @@ -2960,6 +3132,7 @@ static int zram_add(void) > init_rwsem(&zram->init_lock); > #ifdef CONFIG_ZRAM_WRITEBACK > zram->wb_batch_size = 32; > + zram->wb_compressed = false; > #endif > > /* gendisk structure */ > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h > index c6d94501376c..72fdf66c78ab 100644 > --- a/drivers/block/zram/zram_drv.h > +++ b/drivers/block/zram/zram_drv.h > @@ -128,6 +128,7 @@ struct zram { > #ifdef CONFIG_ZRAM_WRITEBACK > struct file *backing_dev; > bool wb_limit_enable; > + bool wb_compressed; > u32 wb_batch_size; > u64 bd_wb_limit; > struct block_device *bdev;