On 4/30/25 20:29, Ryan Roberts wrote: > page_cache_ra_order() takes a parameter called new_order, which is > intended to express the preferred order of the folios that will be > allocated for the readahead operation. Most callers indeed call this > with their preferred new order. But page_cache_async_ra() calls it with > the preferred order of the previous readahead request (actually the > order of the folio that had the readahead marker, which may be smaller > when alignment comes into play). > > And despite the parameter name, page_cache_ra_order() always treats it > at the old order, adding 2 to it on entry. As a result, a cold readahead > always starts with order-2 folios. > > Let's fix this behaviour by always passing in the *new* order. Makes sense. > > Worked example: > > Prior to the change, mmaping an 8MB file and touching each page > sequentially, resulted in the following, where we start with order-2 > folios for the first 128K then ramp up to order-4 for the next 128K, > then get clamped to order-5 for the rest of the file because pa_pages is > limited to 128K: > > TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER > ----- ---------- ---------- --------- ------- ------- ----- ----- > FOLIO 0x00000000 0x00004000 16384 0 4 4 2 > FOLIO 0x00004000 0x00008000 16384 4 8 4 2 > FOLIO 0x00008000 0x0000c000 16384 8 12 4 2 > FOLIO 0x0000c000 0x00010000 16384 12 16 4 2 > FOLIO 0x00010000 0x00014000 16384 16 20 4 2 > FOLIO 0x00014000 0x00018000 16384 20 24 4 2 > FOLIO 0x00018000 0x0001c000 16384 24 28 4 2 > FOLIO 0x0001c000 0x00020000 16384 28 32 4 2 > FOLIO 0x00020000 0x00030000 65536 32 48 16 4 > FOLIO 0x00030000 0x00040000 65536 48 64 16 4 > FOLIO 0x00040000 0x00060000 131072 64 96 32 5 > FOLIO 0x00060000 0x00080000 131072 96 128 32 5 > FOLIO 0x00080000 0x000a0000 131072 128 160 32 5 > FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5 > ... > > After the change, the same operation results in the first 128K being > order-0, then we start ramping up to order-2, -4, and finally get > clamped at order-5: > > TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER > ----- ---------- ---------- --------- ------- ------- ----- ----- > FOLIO 0x00000000 0x00001000 4096 0 1 1 0 > FOLIO 0x00001000 0x00002000 4096 1 2 1 0 > FOLIO 0x00002000 0x00003000 4096 2 3 1 0 > FOLIO 0x00003000 0x00004000 4096 3 4 1 0 > FOLIO 0x00004000 0x00005000 4096 4 5 1 0 > FOLIO 0x00005000 0x00006000 4096 5 6 1 0 > FOLIO 0x00006000 0x00007000 4096 6 7 1 0 > FOLIO 0x00007000 0x00008000 4096 7 8 1 0 > FOLIO 0x00008000 0x00009000 4096 8 9 1 0 > FOLIO 0x00009000 0x0000a000 4096 9 10 1 0 > FOLIO 0x0000a000 0x0000b000 4096 10 11 1 0 > FOLIO 0x0000b000 0x0000c000 4096 11 12 1 0 > FOLIO 0x0000c000 0x0000d000 4096 12 13 1 0 > FOLIO 0x0000d000 0x0000e000 4096 13 14 1 0 > FOLIO 0x0000e000 0x0000f000 4096 14 15 1 0 > FOLIO 0x0000f000 0x00010000 4096 15 16 1 0 > FOLIO 0x00010000 0x00011000 4096 16 17 1 0 > FOLIO 0x00011000 0x00012000 4096 17 18 1 0 > FOLIO 0x00012000 0x00013000 4096 18 19 1 0 > FOLIO 0x00013000 0x00014000 4096 19 20 1 0 > FOLIO 0x00014000 0x00015000 4096 20 21 1 0 > FOLIO 0x00015000 0x00016000 4096 21 22 1 0 > FOLIO 0x00016000 0x00017000 4096 22 23 1 0 > FOLIO 0x00017000 0x00018000 4096 23 24 1 0 > FOLIO 0x00018000 0x00019000 4096 24 25 1 0 > FOLIO 0x00019000 0x0001a000 4096 25 26 1 0 > FOLIO 0x0001a000 0x0001b000 4096 26 27 1 0 > FOLIO 0x0001b000 0x0001c000 4096 27 28 1 0 > FOLIO 0x0001c000 0x0001d000 4096 28 29 1 0 > FOLIO 0x0001d000 0x0001e000 4096 29 30 1 0 > FOLIO 0x0001e000 0x0001f000 4096 30 31 1 0 > FOLIO 0x0001f000 0x00020000 4096 31 32 1 0 > FOLIO 0x00020000 0x00024000 16384 32 36 4 2 > FOLIO 0x00024000 0x00028000 16384 36 40 4 2 > FOLIO 0x00028000 0x0002c000 16384 40 44 4 2 > FOLIO 0x0002c000 0x00030000 16384 44 48 4 2 > FOLIO 0x00030000 0x00034000 16384 48 52 4 2 > FOLIO 0x00034000 0x00038000 16384 52 56 4 2 > FOLIO 0x00038000 0x0003c000 16384 56 60 4 2 > FOLIO 0x0003c000 0x00040000 16384 60 64 4 2 > FOLIO 0x00040000 0x00050000 65536 64 80 16 4 > FOLIO 0x00050000 0x00060000 65536 80 96 16 4 > FOLIO 0x00060000 0x00080000 131072 96 128 32 5 > FOLIO 0x00080000 0x000a0000 131072 128 160 32 5 > FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5 > FOLIO 0x000c0000 0x000e0000 131072 192 224 32 5 I guess performance wise this will be worse than earlier ? Although it does fix the semantics for page_cache_ra_order() with respect to the parameter 'new_order'. > ... > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> > --- > mm/readahead.c | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index 6a4e96b69702..8bb316f5a842 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -479,9 +479,6 @@ void page_cache_ra_order(struct readahead_control *ractl, > > limit = min(limit, index + ra->size - 1); > > - if (new_order < mapping_max_folio_order(mapping)) > - new_order += 2; > - > new_order = min(mapping_max_folio_order(mapping), new_order); > new_order = min_t(unsigned int, new_order, ilog2(ra->size)); > new_order = max(new_order, min_order); > @@ -683,6 +680,7 @@ void page_cache_async_ra(struct readahead_control *ractl, > ra->size = get_next_ra_size(ra, max_pages); > ra->async_size = ra->size; > readit: Should not the earlier conditional check also be brought here before incrementing the order ? Just curious. if (new_order < mapping_max_folio_order(mapping)) > + order += 2; > ractl->_index = ra->start; > page_cache_ra_order(ractl, ra, order); > }
