Re: [Bug 202089] New: transparent hugepage not compatable with madvise(MADV_DONTNEED)

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jan 03, 2019 at 10:44:22AM +0100, Michal Hocko wrote:
> 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:
> > > >   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.

Should the MADV_DONTNEED hint imply MADV_NOHUGEPAGE?  It'd prevent
coalescing elsewhere in the VMA, so that might negatively affect other
programs.