On 05/05/2025 11:09, Anshuman Khandual wrote: > > > 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 ? Maybe, maybe not. If higer order always gave better performance then we would always surely use the highest order? Order-0 is a bit easier to allocate than order-2. So if the file actually isn't being accessed sequentially, allocating order-0 for the cold cache case might actually be better over all? > Although it > does fix the semantics for page_cache_ra_order() with respect to the > parameter 'new_order'. Yes that's the piece I was keen to sort out; Once you get to patch 5 it's important that new_order really does mean new_order otherwise we would end up allocating higher order than the arch intedended. If we think we really *should* be starting at order-2 instead of order-0, we should pass 2 as new_order instead of 0. > >> ... >> >> 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)) No that's not needed. page_cache_ra_order() will clamp new_order appropriately. The conditional that I removed was unneeded becaude the following lines are clamping the new value explicitly anyway. Thanks, Ryan > >> + order += 2; >> ractl->_index = ra->start; >> page_cache_ra_order(ractl, ra, order); >> }
