Hello,
Applied, thanks!
Samuel
Milos Nikic, le dim. 25 oct. 2026 20:19:17 -0700, a ecrit:
> Hello,
>
> First to state a problem:
>
> Ext2fs preallocates up to 8 blocks when a file grows, and marks them
> allocated in the block bitmap while only the node's memory owns them.
> A crash before the file uses them or gives them back leaks them. With
> the journal this matters even more: the bitmap change commits with the
> allocation, and a normal boot only replays the log, without the full
> check that would free the blocks. Killing the translator during
> tar -xf of a Linux tree left such blocks repeatedly.
>
> Fix:
> Patch 1 is the fix the journal needs. It keeps the preallocation
> window in memory only, in a per-group map next to the busy blocks, and
> sets a block in the bitmap only when a file takes it. The layout does
> not change: in the test below, files get exactly the same fragments as
> before (1949 each). Ten kills of the translator at random times during
> rm -rf and tar -xf, and filling the disk with several writers, left
> e2fsck -f clean. Simply not preallocating with a journal, as I first
> tried, also fixes the leak, but doubles the fragments (4097).
>
> Improvement:
> Patch 2 is an improvement on top inspired by ext3, and does not depend
> on the journal. Files written at the same time take blocks in turn,
> so each fragment was at most one 8-block window long.
> A file that keeps using up its window now gets one twice as big, up to
> 1024 blocks, and the window is dropped at the last close instead of whenever
> the inode is written.
> ext3 also doubled its reservation windows, up to 1027 blocks.
>
> Fragments per file, 3 runs each on a fresh 4 KB-block filesystem in an
> i386 VM:
>
> before patch 1 patch 2
> 8 files appended in turn (64 MB) 1949 1949 23
> 4 parallel dd (256 MB) ~6400 ~6300 55-70
>
> The same holds without a journal (23 for the appended files). With
> two concurrent tar -xf of a Linux tree, half as many files are
> fragmented with patch 2 (0.4% against 0.8%), but about 200 more small
> holes of 4 to 64K stay in the free space, 1% of it, mostly what small
> files leave of their windows. The time is the same (about 205 s).
> Capping a window at the file's size made both worse, so it is not in
> the series. A large Debian upgrade with both patches on the root
> filesystem left e2fsck clean.
>
> The patches are independent of other work that is still pending.
>
> Thanks,
> Milos
>
> Milos Nikic (2):
> ext2fs: Keep block preallocation in memory
> ext2fs: Grow preallocation windows for sequential writers
>
> ext2fs/balloc.c | 274 ++++++++++++++++++++++++++++++++--------------
> ext2fs/ext2_fs.h | 3 +
> ext2fs/ext2fs.h | 6 +-
> ext2fs/getblk.c | 70 ++++++------
> ext2fs/ialloc.c | 1 +
> ext2fs/inode.c | 17 ++-
> ext2fs/truncate.c | 4 +-
> 7 files changed, 256 insertions(+), 119 deletions(-)
>
> --
> 2.56.0
>
--
Samuel
«Le monochrome, c'est pour ceux qui s'intéressent (encore) au contenu.
Usenet dans ces conditions, c'est comme le web avec lynx, on prend
trop conscience du vide, c'est déprimant.»
-+- JLC in Guide du linuxien pervers : "Coup de cafard..." -+-
