linux-kernel - Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment

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Message-ID: <CA+KHdyWZ9T2GEuUENXD_OYHX1JxKfZuW5YzmDtqXUYSgkQd8fQ@mail.gmail.com>
Date:   Wed, 29 Sep 2021 16:30:39 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ping Fang <pifang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1

>
> So the idea is that once we run into a dead end because we took a left
> subtree, we rollback to the next possible rigth subtree and try again.
> If we run into another dead end, we repeat ... thus, this can now happen
> more than once.
>
> I assume the only implication is that this can now be slower in some
> corner cases with larger alignment, because it might take longer to find
> something suitable. Fair enough.
>
Yep, your understanding is correct regarding the tree traversal. If no
suitable block
is found in left sub-tree we roll-back and check right one. So it can
be(the scanning)
more than one time.

I did some performance analyzing using vmalloc test suite to figure
out a performance
loss for allocations with specific alignment. On that syntactic test i
see approx. 30%
of degradation:

2.225 microseconds vs 1.496 microseconds. That time includes both
vmalloc() and vfree()
calls. I do not consider it as a big degrade, but from the other hand
we can still adjust the
search length for alignments > one page:

# add it on top of previous proposal and search length instead of size
length = align > PAGE_SIZE ? size + align:size;

in that case we solve a KASAN issue + do not introduce a degrade. For
the PAGE_SIZE
alignment all free blocks are aligned to it anyway. As for users which
uses a fixed range
that is same as a requested size and at the same time want to apply a
special alignment
is not considered as a common case, also we do not have such users.

Thoughts?

> >
> > Could you please help and test the KASAN use case?
>
> Just tried it, works just fine with KASAN and makes sense in general,
> thanks!
>
Good!

Sorry for the delay.

--
Uladzislau Rezki