Re: [PATCH RFC] mm readahead: Fix the readahead fail in case of emptynuma node

[Raghavendra K T]

On 12/14/2013 06:09 AM, Linus Torvalds wrote:
> On Wed, Dec 11, 2013 at 3:05 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > But I'm really struggling to think up an implementation!  The current
> > code looks only at the caller's node and doesn't seem to make much
> > sense.  Should we look at all nodes?  Hard to say without prior
> > knowledge of where those pages will be coming from.
>
> I really think we want to put an upper bound on the read-ahead, and
> I'm not convinced we need to try to be excessively clever about it. We
> also probably don't want to make it too expensive to calculate,
> because afaik this ends up being called for each file we open when we
> don't have pages in the page cache yet.
>
> The current function seems reasonable on a single-node system. Let's
> not kill it entirely just because it has some odd corner-case on
> multi-node systems.
>
> In fact, for all I care, I think it would be perfectly ok to just use
> a truly stupid hard limit ("you can't read-ahead more than 16MB" or
> whatever).
>
> What we do *not* want to allow is to have people call "readahead"
> functions and basically kill the machine because you now have a
> unkillable IO that is insanely big. So I'd much rather limit it too
> much than too little. And on absolutely no sane IO susbsystem does it
> make sense to read ahead insane amounts.
>
> So I'd rather limit it to something stupid and small, than to not
> limit things at all.
>
> Looking at the interface, for example, the natural thing to do for the
> "readahead()" system call, for example, is to just give it a size of
> ~0ul, and let the system limit things, becaue limiting things in useer
> space is just not reasonable.
>
> So I really do *not* think it's fine to just remove the limit entirely.

Very sorry for late reply (was on very loong vacation).

How about having 16MB limit only for remote readaheads and continuing
the rest as is, something like below:

#define MAX_REMOTE_READAHEAD    4096UL

unsigned long max_sane_readahead(unsigned long nr)
{

	unsigned long local_free_page = (node_page_state(numa_node_id(), 
NR_INACTIVE_FILE)
	+ node_page_state(numa_node_id(), NR_FREE_PAGES));
	unsigned long sane_nr = min(nr, MAX_REMOTE_READAHEAD);

	return (local_free_page ? min(nr, local_free_page / 2) : sane_nr);
}

or we can enforce 16MB limit for all the case too.

I 'll send a patch accordingly.

(readahead max will scale accordingly if we dont have 4k page size
 above).

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