On 28 Mar 2025, at 9:09, Ryan Roberts wrote: > On 27/03/2025 20:07, Zi Yan wrote: >> On 27 Mar 2025, at 12:44, Matthew Wilcox wrote: >> >>> On Thu, Mar 27, 2025 at 04:06:58PM +0000, Ryan Roberts wrote: >>>> So let's special-case the read(ahead) logic for executable mappings. The >>>> trade-off is performance improvement (due to more efficient storage of >>>> the translations in iTLB) vs potential read amplification (due to >>>> reading too much data around the fault which won't be used), and the >>>> latter is independent of base page size. I've chosen 64K folio size for >>>> arm64 which benefits both the 4K and 16K base page size configs and >>>> shouldn't lead to any read amplification in practice since the old >>>> read-around path was (usually) reading blocks of 128K. I don't >>>> anticipate any write amplification because text is always RO. >>> >>> Is there not also the potential for wasted memory due to ELF alignment? >>> Kalesh talked about it in the MM BOF at the same time that Ted and I >>> were discussing it in the FS BOF. Some coordination required (like >>> maybe Kalesh could have mentioned it to me rathere than assuming I'd be >>> there?) >>> >>>> +#define arch_exec_folio_order() ilog2(SZ_64K >> PAGE_SHIFT) >>> >>> I don't think the "arch" really adds much value here. >>> >>> #define exec_folio_order() get_order(SZ_64K) >> >> How about AMD’s PTE coalescing, which does PTE compression at >> 16KB or 32KB level? It covers 4 16KB and 2 32KB, at least it will >> not hurt AMD PTE coalescing. Starting with 64KB across all arch >> might be simpler to see the performance impact. Just a comment, >> no objection. :) > > exec_folio_order() is defined per-architecture and SZ_64K is the arm64 preferred > size. At the moment x86 is not opted in, but they could choose to opt in with > 32K (or whatever else makese sense) if the HW supports coalescing. Oh, I missed that part. I thought, since arch_ is not there, it was the same for all arch. > > I'm not sure if you thought this was global and are arguing against that, or if > you are arguing for it to be global because it will more easily show us > performance regressions earlier if x86 is doing this too? I thought it was global. It might be OK to set it global and let different arch to optimize it as it rolls out. Opt-in might be "never" until someone looks into it, but if it is global and it changes performance, people will notice and look into it. -- Best Regards, Yan, Zi
