Re: [PATCH v2] fix judgment error in shmem_is_huge()

[Hugh Dickins <hughd@google.com>]

On Fri, 24 Sep 2021, Hugh Dickins wrote:
> On Thu, 9 Sep 2021, Liu Yuntao wrote:
> 
> > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded
> > up correctly. When the page index points to the first page in a huge
> > page, round_up() cannot bring it to the end of the huge page, but
> > to the end of the previous one.
> > 
> > an example:
> > HPAGE_PMD_NR on my machine is 512(2 MB huge page size).
> > After allcoating a 3000 KB buffer, I access it at location 2050 KB.
> 
> Your example is certainly helpful, but weird!  It's not impossible,
> but wouldn't it be easier to understand if you said "2048 KB" there?
> 
> > In shmem_is_huge(), the corresponding index happens to be 512.
> > After rounded up by HPAGE_PMD_NR, it will still be 512 which is
> > smaller than i_size, and shmem_is_huge() will return true.
> > As a result, my buffer takes an additional huge page, and that
> > shouldn't happen when shmem_enabled is set to within_size.
> 
> A colleague very recently opened my eyes to within_size on shmem_enabled:
> I've always been dubious of both, but they can work quite well together.
> 
> > 
> > Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> > Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com>
> 
> Thanks, with a nice simplification from Kirill.
> 
> Acked-by: Hugh Dickins <hughd@google.com>

Andrew has just sent this on to Linus - thanks - and that's fine:
no need to get in the way of that.

But since replying, I have remembered more history, and there is
something that we need to be aware of.

Whereas to you this is a straightforward off-by-one (or off-by-page)
fix, it also results in a significant change in behaviour - I'd say
usually for the better, but some might be surprised.  This patch has
Kirill's Ack and my Ack, and I hope and believe that we can get away
with the change in behaviour: but let's be aware of it.

The change that concerns me is when, for example, copying a large
file into a huge=within_size tmpfs (or more generally, just writing
to the file by appending at EOF in the usual way).

With the old WITHIN_SIZE code, the first 2MB was allocated in small
pages, then subsequent 2MB extents were allocated with huge pages;
including the final extent, even if it only needed a single byte.

I always thought that was very clunky behaviour, the small pages
coming at the wrong end of the file; and that's why I was dubious
about it as a sensible filesystem mount option.  But I was under
the impression that it was the intended behaviour.

With your new WITHIN_SIZE code, all those appending allocations
are outside i_size, and the whole file is allocated in small pages.
(Then maybe later on khugepaged can assemble huge pages for it.)

Your patch makes within_size more sensible than it was for pre-sized
files (and I think it's fair to say that the majority of files in
shmem's internal mount, subject to thp/shmem_enabled, are likely to
be fixed-size files); and better-defined than it used to be on
growing files, but they won't get the huge pages they used to.

Hugh