On Mon, 3 Feb 2025 22:55:29 +0000 Usama Arif <usamaarif642@xxxxxxxxx> wrote: > This is introduced for larger folios. If a large folio has subpages > present in multiple regions, it will be considered multiple times. > This can be when checking access I think this behavior makes sense. If a 2 MiB address range is known to be accessed or not, treating DAMON regions in the address range as all accessed or not aligns with the DAMON's region definition and makes sense in general, to me. Note that DAMON's adaptive regions adjustment mechanism will make the DAMON regions in a large folio to be merged in near future. > or applying DAMOS schemes. For e.g. > in pa_stat, folios split across N regions will be counted N times, > giving inaccurate results. Nice catch! I agree this could be a problem. > Hence, only consider a page for access check/DAMOS scheme only if the head > page is part of that region as well. For access checking, this seems wrong to me. This change will unnecessarily mark some DAMON regions as not accessed. Even for DAMOS, this seems not very optimum. 1. DAMOS will unnecessarily repeat PAGE_SIZE skippings on second and later DAMON regions of a same large folio. 2. If only a small part of the large folio belongs to the first DAMON region, and the DAMON region is not eligible to the scheme, while the second region is, the scheme action will not applied to the second region. Also, I think this problem can happen on vaddr case. Hence, making the change on core layer makes sense to me. That is, let damon_ops->apply_scheme() inform the caller (damos_apply_scheme()) how much bytes of the next region should be ignored at applying the action. Due to the complicated implementation of DAMOS core logic, this might be not very simple. I think the additional complexity might outweigh the benefit for following reasons. First, adaptive regions adjustment mechanism will make DAMON regions in same large folios to be merged soon. So the problem will be not significant in common case. Second, any threads could collapse any parts of memory into large folios while DAMOS is traversing DAMON regions. So this problem cannot perfectly be solved unless we make DAMOS' DAMON regions traversal exclusive with any large folios collapsing. Which would add more complexity. And given DAMON's best-effort nature, keeping the issue until we get a simple and effective solution is ok to me, unless a real significant issue from this is confirmed. Usama, what do you think? Thanks, SJ [...]
