Re: reiser4: FITRIM ioctl -- how to grab the space?

[Edward Shishkin <edward.shishkin@gmail.com>]

grab_space() is only for processes which require disk space (including
wandered blocks). For example ->read() modifies a superblock (atime),
which should be written via journal. So ->read() grab 1 block, etc.

As to FITRIM ioctl: I don't think it needs to reserve disk space at all: 
it will
issue only discard requests.

Edward.


On 07/31/2014 10:47 PM, Ivan Shapovalov wrote:
> ...and here is volume 2 of my neverending question series :)
>
> A stub ioctl, placed as I described, seems to work and log a warning each time
> I invoke `fstrim` on anything mounted.
>
> Now on to space allocation. This seems pretty easy in the first approximation:
> reiser4_alloc_blocks_bitmap() seems to do what's needed, finding the nearest
> free extent, not the biggest one. So we could just call reiser4_alloc_blocks()
> plus reiser4_dealloc_blocks(BA_DEFER) repeatedly, while allocations succeed.
>
> However, I suppose we need to grab all free space before proceeding. There is
> no primitive for grabbing all free space, so we need to read block counters,
> calculate amount of space to grab and then call reiser4_grab_reserved()
> (as we want to allocate everything, including the reserved space. This creates
> two problems (as I see it):
> - there is a race between reading block counters (under sb spinlock) and
>    performing the grab (which takes the spinlock on its own);
> - if anything will try to grab space during discarding, it will get an ENOSPC,
>    while it's better to make the process wait until discarding is completed.
>
> I'm not sure whether the last problem really exists (there is BA_CAN_COMMIT,
> but I don't know whether is it used consistently where possible).
>
> Could you explain these things?
>
> Thanks,