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SubjectExt3 -mm reservations code: is this fix really correct?
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Date
Hi,

In ext3-reservations-window-allocation-fix.patch from -mm, we try to
make sure that we always search for a new reservation from the goal
forwards, not just from the previous window forwards. I'm assuming this
is done to optimise random writes.

I'm still not convinced we get it right. In alloc_new_reservation(), we
do:

if ((my_rsv->rsv_start <= group_end_block) &&
(my_rsv->rsv_end > group_end_block))
return -1;
We get into alloc_new_reservation in the first place either when the
goal is outside the window, or we could not allocate inside the window.

Now, in the latter case, the check is correct --- if the window spans
two block groups and we couldn't allocate in the first block group, then
we should continue in the next one.

But what if the goal was in the current block group, but was *prior* to
the window? The goal is outside the window, yet the above check may
still be true, and we'll incorrectly decide to avoid the current block
group entirely.

I think we need an "&& start_block >= my_rsv->rsv_start" to deal with
this.

If we get past that test --- the reservation window is all entirely
inside one group --- then we have the following chunk in
xt3-reservations-window-allocation-fix.patch:

- /* remember where we are before we discard the old one */
- if (my_rsv->rsv_end + 1 > start_block)
- start_block = my_rsv->rsv_end + 1;
- search_head = my_rsv;
+ search_head = search_reserve_window(&my_rsv->rsv_node, start_block);
which I'm assuming is trying to pin the search start to the goal block.
But that's wrong --- search_reserve_window does a downwards-traversing
tree search, and needs to be given the root of the tree in order to find
a given block. Giving it the current reservation node as the search
start point is not going to allow it to find the right node in all
cases.

Have I misunderstood something?

Fortunately, none of the above should affect the normal hot path of
sequential allocation, but it may well penalise random writes.

Cheers,
Stephen

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