On Sun, 26 Sep 2021, liuyuntao wrote: > On Sat, 25 Sep 2021, Hugh Dickins wrote: > > 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? > > I wanted to emphasize that access to any bit in the first page will > trigger this problem, so I didn't use "2048 KB". Okay, thanks, I see your point now. (And I have to admit that, in my confusion, I had thought 2050 KB would be index 514 - of course not!) > > > > In shmem_is_huge(), the corresponding index happens to be 512. ... > > 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. > > Although my patch changes shmem's behaviour, it makes shmem consistent > with the documentation. I think with the new code, it will be easier > for our users to understand. Yes, I do agree with you. But the change in behaviour when appending at EOF is significant, and needed to be called out - I think none of quite realized that effect at first. Hugh
