Re: [PATCH v2 2/4] mm: replace exec_folio_order() with generic preferred_exec_order()

[Jan Kara <jack@suse.cz>]

On Thu 26-03-26 08:40:21, Usama Arif wrote:
> On 20/03/2026 17:42, Jan Kara wrote:
> >> +	/* Step down under memory pressure */
> >> +	gfp = mapping_gfp_mask(vma->vm_file->f_mapping);
> >> +	zone = first_zones_zonelist(node_zonelist(numa_node_id(), gfp),
> >> +				    gfp_zone(gfp), NULL)->zone;
> >> +	if (zone) {
> >> +		while (order > 0 &&
> >> +		       !zone_watermark_ok(zone, order,
> >> +					  high_wmark_pages(zone), 0, 0))
> >> +			order--;
> >> +	}
> > 
> > It looks wrong for this logic to be here. Trimming order based on memory
> > pressure makes sense (and we've already got reports that on memory limited
> > devices large order folios in the page cache have too big memory overhead
> > so we'll likely need to handle that for page cache allocations in general)
> > but IMHO it belongs to page_cache_ra_order() or some other common place
> > like that.
> > 
> > 								Honza
> 
> So I have been thinking about this. readahead_gfp_mask() already sets
> __GFP_NORETRY, so we wont try aggressive reclaim/compaction to satisfy
> the allocation. page_cache_ra_order() falls through to the fallback path
> faulting in order 0 page when allocation is not satsified.
> 
> So the allocator already naturally steps down under memory pressure,
> the explicit zone_watermark_ok() loop might be redundant?

Probably yes. I still think we'll have to somehow better tweak the used
order based on expected size of the page cache (2M folios seem unreasonably
large for a machine that has e.g. 1G of memory in total, even if it has
enough free memory at this point in time - we'll benefit from smaller
folios and thus finer grained folio activity tracking for such cases). But
that's not for this patch set.

> What are your thoughts on just setting
> ra->order = min(HPAGE_PMD_ORDER, ilog2(SZ_2M >> PAGE_SHIFT))?
> We can do the higher orlder allocation with gfp &= ~__GFP_RECLAIM
> for the VM_EXEC case.

Yes, it's simple and it makes sense to me so if others are fine with it...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR