Re: [RFC PATCH v1] mm/filemap: Allow arch to request folio size for exec memory

[Matthew Wilcox]

On Sat, Jan 13, 2024 at 11:54:23AM +1300, Barry Song wrote:
> > > Perhaps an alternative would be to double ra->size and set ra->async_size to
> > > (ra->size / 2)? That would ensure we always have 64K aligned blocks but would
> > > give us an async portion so readahead can still happen.
> >
> > this might be worth to try as PMD is exactly doing this because async
> > can decrease
> > the latency of subsequent page faults.
> >
> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >         /* Use the readahead code, even if readahead is disabled */
> >         if (vm_flags & VM_HUGEPAGE) {
> >                 fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> >                 ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
> >                 ra->size = HPAGE_PMD_NR;
> >                 /*
> >                  * Fetch two PMD folios, so we get the chance to actually
> >                  * readahead, unless we've been told not to.
> >                  */
> >                 if (!(vm_flags & VM_RAND_READ))
> >                         ra->size *= 2;
> >                 ra->async_size = HPAGE_PMD_NR;
> >                 page_cache_ra_order(&ractl, ra, HPAGE_PMD_ORDER);
> >                 return fpin;
> >         }
> > #endif
> >
> 
> BTW, rather than simply always reading backwards,  we did something very
> "ugly" to simulate "read-around" for CONT-PTE exec before[1]
> 
> if page faults happen in the first half of cont-pte, we read this 64KiB
> and its previous 64KiB. otherwise, we read it and its next 64KiB.

I don't think that makes sense.  The CPU executes instructions forwards,
not "around".  I honestly think we should treat text as "random access"
because function A calls function B and functions A and B might well be
very far apart from each other.  The only time I see you actually
getting "readahead" hits is if a function is split across two pages (for
whatever size of page), but that's a false hit!  The function is not,
generally, 64kB long, so doing readahead is no more likely to bring in
the next page of text that we want than reading any other random page.

Unless somebody finds the GNU Rope source code from 1998, or recreates it:
https://lwn.net/1998/1029/als/rope.html
Then we might actually have some locality.

Did you actually benchmark what you did?  Is there really some locality
between the code at offset 256-288kB in the file and then in the range
192kB-256kB?