On Mon, 24 Apr 2023, Charan Teja Kalla wrote: > On 4/21/2023 5:37 AM, Hugh Dickins wrote: > > This is where I ran out of time. I'm afraid all the focus on > > fadvise_calc_endbyte() has distracted you from looking at the DONTNEED > > in mm/fadvise.c: where there are detailed comments on why and how it > > then narrows the DONTNEED range. And aside from needing to duplicate > > that here for shmem (or put it into another or combined helper), it > > implies to me that shmem_isolate_pages_range() needs to do a similar > > narrowing, when it finds that the range overlaps part of a large folio. > > > Sure, will include those range calculations for shmem pages too. Oh, I forgot this issue, you would have liked me to look at V8 by now, to see whether I agree with your resolution there. Sorry, no, I've not been able to divert my concentration to it yet. And it's quite likely that I shall disagree, because I've a history of disagreeing even with myself on such range widening/narrowing issues - reconciling conflicting precedents is difficult :( > > > Something that has crossed my mind as a worry, but I've not had time > > to look further into (maybe it's no concern at all) is the question > > of this syscall temporarily isolating a very large number of folios, > > whether they need to be (or perhaps already are) counted in > > NR_ISOLATED_ANON, whether too many isolated needs to be limited. > > They are _not_ counted as ISOLATED_ANON now as this operation is for a > small duration. I do see there exists too_many_isolated() checks in > direct reclaim/compaction logic where it is necessary to stop the > multiple processes in the direct reclaim from isolating too many pages. > > I am not able to envisage such problem here, where usually single > process doing the fadvise operation on a file. Even If the file is > opened by multiple processes and do fadvise, the operation is limited > only to the pages of this file and doesn't impact the system. > > Please let me know if I'm missing something where I should be counting > these as NR_ISOLATED. Please grep for NR_ISOLATED, to see where and how they get manipulated already, and follow the existing examples. The case that sticks in my mind is in mm/mempolicy.c, where the migrate_pages() syscall can build up a gigantic quantity of transiently isolated pages: your syscall can do the same, so should account for itself in the same way. I'm not claiming that mm/vmscan.c's too_many_isolated(), and the way it gets used by shrink_inactive_list(), is perfect: not at all. But please follow existing convention. Sorry, that's all for now. Hugh
