On 10 Apr 2026, at 11:06, Andres Freund wrote: > Hi, > > On 2026-03-26 21:42:50 -0400, Zi Yan wrote: >> Replace it with a check on the max folio order of the file's address space >> mapping, making sure PMD_ORDER is supported. > > Question from a user space developer (who learned of this via LWN [1]): > > Right now MADV_COlLAPSE on file mappings appears to only only work with > CONFIG_READ_ONLY_THP_FOR_FS. > > That's pretty sad, because I see quite consistent gains (~15%) from doing > madvise(MADV_COLLAPSE) on e.g. postgres' .text - but because > CONFIG_READ_ONLY_THP_FOR_FS is not enabled default (or at least not commonly, > I think?) in distro kernels, it never quite seemed worth the fiddliness. > > > Do I understand correctly that by replacing the check here (and the > predecessor commits), madvise(MADV_COLLAPSE) on file mappings might actually > start working in more situations on typical distro kernels?
Yes, assuming your file system supports PMD THPs. Based on[1], ext4 and xfs have the support, btrfs supports large folio but not up to PMD order. I have not checked other FSes supporting large folio[2]. [1] https://lore.kernel.org/all/[email protected]/ [2] https://lore.kernel.org/all/[email protected]/ Best Regards, Yan, Zi
