Re: [RFC 7/7] mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER

[Minchan Kim <minchan@kernel.org>]

On Mon, May 20, 2019 at 11:28:01AM +0200, Michal Hocko wrote:
> [cc linux-api]
> 
> On Mon 20-05-19 12:52:54, Minchan Kim wrote:
> > System could have much faster swap device like zRAM. In that case, swapping
> > is extremely cheaper than file-IO on the low-end storage.
> > In this configuration, userspace could handle different strategy for each
> > kinds of vma. IOW, they want to reclaim anonymous pages by MADV_COLD
> > while it keeps file-backed pages in inactive LRU by MADV_COOL because
> > file IO is more expensive in this case so want to keep them in memory
> > until memory pressure happens.
> > 
> > To support such strategy easier, this patch introduces
> > MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER options in madvise(2) like
> > that /proc/<pid>/clear_refs already has supported same filters.
> > They are filters could be Ored with other existing hints using top two bits
> > of (int behavior).
> 
> madvise operates on top of ranges and it is quite trivial to do the
> filtering from the userspace so why do we need any additional filtering?
> 
> > Once either of them is set, the hint could affect only the interested vma
> > either anonymous or file-backed.
> > 
> > With that, user could call a process_madvise syscall simply with a entire
> > range(0x0 - 0xFFFFFFFFFFFFFFFF) but either of MADV_ANONYMOUS_FILTER and
> > MADV_FILE_FILTER so there is no need to call the syscall range by range.
> 
> OK, so here is the reason you want that. The immediate question is why
> cannot the monitor do the filtering from the userspace. Slightly more
> work, all right, but less of an API to expose and that itself is a
> strong argument against.

What I should do if we don't have such filter option is to enumerate all of
vma via /proc/<pid>/maps and then parse every ranges and inode from string,
which would be painful for 2000+ vmas.

> 
> > * from v1r2
> >   * use consistent check with clear_refs to identify anon/file vma - surenb
> > 
> > * from v1r1
> >   * use naming "filter" for new madvise option - dancol
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  include/uapi/asm-generic/mman-common.h |  5 +++++
> >  mm/madvise.c                           | 14 ++++++++++++++
> >  2 files changed, 19 insertions(+)
> > 
> > diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> > index b8e230de84a6..be59a1b90284 100644
> > --- a/include/uapi/asm-generic/mman-common.h
> > +++ b/include/uapi/asm-generic/mman-common.h
> > @@ -66,6 +66,11 @@
> >  #define MADV_WIPEONFORK 18		/* Zero memory on fork, child only */
> >  #define MADV_KEEPONFORK 19		/* Undo MADV_WIPEONFORK */
> >  
> > +#define MADV_BEHAVIOR_MASK (~(MADV_ANONYMOUS_FILTER|MADV_FILE_FILTER))
> > +
> > +#define MADV_ANONYMOUS_FILTER	(1<<31)	/* works for only anonymous vma */
> > +#define MADV_FILE_FILTER	(1<<30)	/* works for only file-backed vma */
> > +
> >  /* compatibility flags */
> >  #define MAP_FILE	0
> >  
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index f4f569dac2bd..116131243540 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1002,7 +1002,15 @@ static int madvise_core(struct task_struct *tsk, unsigned long start,
> >  	int write;
> >  	size_t len;
> >  	struct blk_plug plug;
> > +	bool anon_only, file_only;
> >  
> > +	anon_only = behavior & MADV_ANONYMOUS_FILTER;
> > +	file_only = behavior & MADV_FILE_FILTER;
> > +
> > +	if (anon_only && file_only)
> > +		return error;
> > +
> > +	behavior = behavior & MADV_BEHAVIOR_MASK;
> >  	if (!madvise_behavior_valid(behavior))
> >  		return error;
> >  
> > @@ -1067,12 +1075,18 @@ static int madvise_core(struct task_struct *tsk, unsigned long start,
> >  		if (end < tmp)
> >  			tmp = end;
> >  
> > +		if (anon_only && vma->vm_file)
> > +			goto next;
> > +		if (file_only && !vma->vm_file)
> > +			goto next;
> > +
> >  		/* Here vma->vm_start <= start < tmp <= (end|vma->vm_end). */
> >  		error = madvise_vma(tsk, vma, &prev, start, tmp,
> >  					behavior, &pages);
> >  		if (error)
> >  			goto out;
> >  		*nr_pages += pages;
> > +next:
> >  		start = tmp;
> >  		if (prev && start < prev->vm_end)
> >  			start = prev->vm_end;
> > -- 
> > 2.21.0.1020.gf2820cf01a-goog
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs