> On Sun, Dec 01, 2024 at 08:57:07PM GMT, Matthew Wilcox wrote: > > Right, mapping with the larger size than needed is one option we're > > considering. But there are few arguments against that: > > > > * Folks are wary of unnecessary large shared mappings, since in the past > > there were issues with OOM killer making unfavorable to postgres > > decisions because of that. It might have changed over time, but to > > confirm that will require some investigation. > > > > * It can cause memory accounting problems. E.g. if we use hugetlb inside > > a cgroup with reservation limits set (something like > > hugetlb.2MB.rsvd.limit_in_bytes), then such mmap() will be counted > > against the limit, even though the memory wasn't allocated -- meaning > > that we claim some resource without using it. > > If it does turn out to be a problem, you can use a similar trick to how > ld.so maps binaries: > > mmap(NULL, 2055640, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f221a758000 > mmap(0x7f221a780000, 1462272, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x28000) = 0x7f221a780000 > mmap(0x7f221a8e5000, 352256, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x18d000) = 0x7f221a8e5000 > mmap(0x7f221a93b000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e2000) = 0x7f221a93b000 > mmap(0x7f221a941000, 52696, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f221a941000 > > Although you wouldn't want to do consecutive mmaps, you'd want to use > mremap() with MREMAP_FIXED -- not to change new_address, but to expand > length over the initial reserving-space mapping. Hm, I don't follow how would that help? From what I understand the suggestion is to have an initial mapping to "reserve" the space, right? But this initial mapping would also be a subject of reservation limits, mentioned above. I was originally experimenting with that, "reserving" some mapping space with PROT_NONE, then slicing off chunks of it for real usage -- but in case of hugetlb and a cgroup it was accounting against the reservation limits for huge pages.
