Re: [RFC PATCH] mm/readahead: readahead aggressively if read drops in willneed range

[Matthew Wilcox]

On Sun, Jan 28, 2024 at 06:12:29PM -0500, Mike Snitzer wrote:
> On Sun, Jan 28 2024 at  5:02P -0500,
> Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> > On Sun, Jan 28, 2024 at 10:25:22PM +0800, Ming Lei wrote:
> > > Since commit 6d2be915e589 ("mm/readahead.c: fix readahead failure for
> > > memoryless NUMA nodes and limit readahead max_pages"), ADV_WILLNEED
> > > only tries to readahead 512 pages, and the remained part in the advised
> > > range fallback on normal readahead.
> > 
> > Does the MAINTAINERS file mean nothing any more?
> 
> "Ming, please use scripts/get_maintainer.pl when submitting patches."

That's an appropriate response to a new contributor, sure.  Ming has
been submitting patches since, what, 2008?  Surely they know how to
submit patches by now.

> I agree this patch's header could've worked harder to establish the
> problem that it fixes.  But I'll now take a crack at backfilling the
> regression report that motivated this patch be developed:

Thank you.

> Linux 3.14 was the last kernel to allow madvise (MADV_WILLNEED)
> allowed mmap'ing a file more optimally if read_ahead_kb < max_sectors_kb.
> 
> Ths regressed with commit 6d2be915e589 (so Linux 3.15) such that
> mounting XFS on a device with read_ahead_kb=64 and max_sectors_kb=1024
> and running this reproducer against a 2G file will take ~5x longer
> (depending on the system's capabilities), mmap_load_test.java follows:
> 
> import java.nio.ByteBuffer;
> import java.nio.ByteOrder;
> import java.io.RandomAccessFile;
> import java.nio.MappedByteBuffer;
> import java.nio.channels.FileChannel;
> import java.io.File;
> import java.io.FileNotFoundException;
> import java.io.IOException;
> 
> public class mmap_load_test {
> 
>         public static void main(String[] args) throws FileNotFoundException, IOException, InterruptedException {
> 		if (args.length == 0) {
> 			System.out.println("Please provide a file");
> 			System.exit(0);
> 		}
> 		FileChannel fc = new RandomAccessFile(new File(args[0]), "rw").getChannel();
> 		MappedByteBuffer mem = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());
> 
> 		System.out.println("Loading the file");
> 
> 		long startTime = System.currentTimeMillis();
> 		mem.load();
> 		long endTime = System.currentTimeMillis();
> 		System.out.println("Done! Loading took " + (endTime-startTime) + " ms");
> 		
> 	}
> }

It's good to have the original reproducer.  The unfortunate part is
that being at such a high level, it doesn't really show what syscalls
the library makes on behalf of the application.  I'll take your word
for it that it calls madvise(MADV_WILLNEED).  An strace might not go
amiss.

> reproduce with:
> 
> javac mmap_load_test.java
> echo 64 > /sys/block/sda/queue/read_ahead_kb
> echo 1024 > /sys/block/sda/queue/max_sectors_kb
> mkfs.xfs /dev/sda
> mount /dev/sda /mnt/test
> dd if=/dev/zero of=/mnt/test/2G_file bs=1024k count=2000
> 
> echo 3 > /proc/sys/vm/drop_caches

(I prefer to unmount/mount /mnt/test; it drops the cache for
/mnt/test/2G_file without affecting the rest of the system)

> java mmap_load_test /mnt/test/2G_file
> 
> Without a fix, like the patch Ming provided, iostat will show rareq-sz
> is 64 rather than ~1024.

Understood.  But ... the application is asking for as much readahead as
possible, and the sysadmin has said "Don't readahead more than 64kB at
a time".  So why will we not get a bug report in 1-15 years time saying
"I put a limit on readahead and the kernel is ignoring it"?  I think
typically we allow the sysadmin to override application requests,
don't we?

> > > @@ -972,6 +974,7 @@ struct file_ra_state {
> > >  	unsigned int ra_pages;
> > >  	unsigned int mmap_miss;
> > >  	loff_t prev_pos;
> > > +	struct maple_tree *need_mt;
> > 
> > No.  Embed the struct maple tree.  Don't allocate it.
> 
> Constructive feedback, thanks.
> 
> > What made you think this was the right approach?
> 
> But then you closed with an attack, rather than inform Ming and/or
> others why you feel so strongly, e.g.: Best to keep memory used for
> file_ra_state contiguous.

That's not an attack, it's a genuine question.  Is there somewhere else
doing it wrong that Ming copied from?  Does the documentation need to
be clearer?  I can't fix what I don't know.