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4f993264fe
As James Z reported in bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=213877 [1.] One-line summary of the problem: Mount multiple SMR block devices exceed certain number cause system non-response [2.] Full description of the problem/report: Created some F2FS on SMR devices (mkfs.f2fs -m), then mounted in sequence. Each device is the same Model: HGST HSH721414AL (Size 14TB). Empirically, found that when the amount of SMR device * 1.5Gb > System RAM, the system ran out of memory and hung. No dmesg output. For example, 24 SMR Disk need 24*1.5GB = 36GB. A system with 32G RAM can only mount 21 devices, the 22nd device will be a reproducible cause of system hang. The number of SMR devices with other FS mounted on this system does not interfere with the result above. [3.] Keywords (i.e., modules, networking, kernel): F2FS, SMR, Memory [4.] Kernel information [4.1.] Kernel version (uname -a): Linux 5.13.4-200.fc34.x86_64 #1 SMP Tue Jul 20 20:27:29 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux [4.2.] Kernel .config file: Default Fedora 34 with f2fs-tools-1.14.0-2.fc34.x86_64 [5.] Most recent kernel version which did not have the bug: None [6.] Output of Oops.. message (if applicable) with symbolic information resolved (see Documentation/admin-guide/oops-tracing.rst) None [7.] A small shell script or example program which triggers the problem (if possible) mount /dev/sdX /mnt/0X [8.] Memory consumption With 24 * 14T SMR Block device with F2FS free -g total used free shared buff/cache available Mem: 46 36 0 0 10 10 Swap: 0 0 0 With 3 * 14T SMR Block device with F2FS free -g total used free shared buff/cache available Mem: 7 5 0 0 1 1 Swap: 7 0 7 The root cause is, there are three bitmaps: - cur_valid_map - ckpt_valid_map - discard_map and each of them will cost ~500MB memory, {cur, ckpt}_valid_map are necessary, but discard_map is optional, since this bitmap will only be useful in mountpoint that small discard is enabled. For a blkzoned device such as SMR or ZNS devices, f2fs will only issue discard for a section(zone) when all blocks of that section are invalid, so, for such device, we don't need small discard functionality at all. This patch introduces a new mountoption "discard_unit=block|segment| section" to support issuing discard with different basic unit which is aligned to block, segment or section, so that user can specify "discard_unit=segment" or "discard_unit=section" to disable small discard functionality. Note that this mount option can not be changed by remount() due to related metadata need to be initialized during mount(). In order to save memory, let's use "discard_unit=section" for blkzoned device by default. Signed-off-by: Chao Yu <chao@kernel.org> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org> |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.