Re: Write-back from inside FS - need suggestions

[Artem Bityutskiy <dedekind@yandex.ru>]

Andrew Morton wrote:
> This is precisely the problem which needs to be solved for delayed
> allocation on ext2/3/4.  This is because it is infeasible to work out how
> much disk space an ext2 pagecache page will take to write out (it will
> require zero to three indirect blocks as well).
>
> When I did delalloc-for-ext2, umm, six years ago I did
> maximally-pessimistic in-memory space accounting and I think I just ran a
> superblock-wide sync operation when ENOSPC was about to happen.  That
> caused all the pessimistic reservations to be collapsed into real ones,
> releasing space.  So as the disk neared a real ENOSPC, the syncs becaome
> more frequent.  But the overhead was small.
>
> I expect that a similar thing was done in the ext4 delayed allocation
> patches - you should take a look at that and see what can be
> shared/generalised/etc.
>
> ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/
>
> Although, judging by the comment in here:
>
> ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/ext4-patches/LATEST/broken-out/ext4-delayed-allocation.patch
>
> + * TODO:
> + *   MUST:
> + *     - flush dirty pages in -ENOSPC case in order to free reserved blocks
>
> things need a bit more work.  Hopefully that's a dead comment.
>
> <looks>
>
> omigod, that thing has gone and done a clone-and-own on half the VFS.
> Anyway, I doubt if you'll be able to find a design description anyway
> but you should spend some time picking it apart.  It is the same problem..

(For some reasons I haven't got your answer in my mailbox, found it in
archives)

Thank you for these pointers. I was looking at ext4 code and found haven't
found what they do in these cases. I think I need some hints to realize
what's going on there. Our FS is so different from traditional ones
- e.g., we do not use buffer heads, we do not have block device
underneath, etc, so I even doubt I can borrow anything from ext4.

I have impression that I just have to implement my own list of
inodes and my own victim-picking policies. Although I still think it
should better be done on VFS level, because it has all these LRU lists,
and I'd duplicate things.

Nevertheless, I add Teo on CC in a hope he'll give me some pointers.

--
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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