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@xxxxxxxxxx> > > Thanks, with a nice simplification from Kirill. > > Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> 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