On Sun 30-12-18 01:48:43, Kirill A. Shutemov wrote: > On Sat, Dec 29, 2018 at 12:53:16PM -0800, Andrew Morton wrote: > > > > (switched to email. Please respond via emailed reply-to-all, not via the > > bugzilla web interface). > > > > On Sat, 29 Dec 2018 09:00:22 +0000 bugzilla-daemon@xxxxxxxxxxxxxxxxxxx wrote: > > > > > https://bugzilla.kernel.org/show_bug.cgi?id=202089 > > > > > > Bug ID: 202089 > > > Summary: transparent hugepage not compatable with > > > madvise(MADV_DONTNEED) > > > Product: Memory Management > > > Version: 2.5 > > > Kernel Version: 4.4.0-117 > > > Hardware: x86-64 > > > OS: Linux > > > Tree: Mainline > > > Status: NEW > > > Severity: high > > > Priority: P1 > > > Component: Other > > > Assignee: akpm@xxxxxxxxxxxxxxxxxxxx > > > Reporter: jianpanlanyue@xxxxxxx > > > Regression: No > > > > > > environment: > > > 1.kernel 4.4.0 on x86_64 > > > 2.echo always > /sys/kernel/mm/transparent_hugepage/enable > > > echo always > /sys/kernel/mm/transparent_hugepage/defrag > > > echo 2000000 > /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan > > > ( faster defrag pages to reproduce problem) > > > > > > problem: > > > 1. use mmap() to allocate 4096 bytes for 1024*512 times (4096*1024*512=2G). > > > 2. use madvise(MADV_DONTNEED) to free most of the above pages, but reserve a > > > few pages(by if（i%33==0) continue;), then process's physical memory firstly > > > come down, but after a few seconds, it rise back to 2G again, and can't come > > > down forever. > > > 3. if i delete this condition(if（i%33==0) continue;) or disable > > > transparent_hugepage by setting 'enable' and 'defrag' to never, all go well and > > > the physical memory can come down expectly. > > > > > > It seems like transparent_hugepage has problems with non-contiguous > > > madvise(MADV_DONTEED). > > It's expected behaviour. > > MADV_DONTNEED doesn't guarantee that the range will not be repopulated > (with or without direct action on application behalf). It's just a hint > for the kernel. I agree with Kirill here but I would be interested in the underlying usecase that triggered this. The test case is clearly artificial but is any userspace actually relying on MADV_DONTNEED reducing the rss longterm? > For sparse mappings, consider using MADV_NOHUGEPAGE. Yes or use a high threshold for khugepaged for collapsing. -- Michal Hocko SUSE Labs
