On 11/30/20 23:51, Johannes Thumshirn wrote:

quoted

Why add an extra function call overhead in the hot path for each I/O ?

At least in my compilation (gcc 10.1) there's now extra function call overhead.
__bio_iov_append_get_pages() get's fully inlined into bio_iov_iter_get_pages().

$ make block/bio.s
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CC      block/bio.s
$ grep __bio_iov_append_get_pages block/bio.s
$ grep bio_iov_iter_get_pages block/bio.s
__kstrtab_bio_iov_iter_get_pages:
        .asciz  "bio_iov_iter_get_pages"
__kstrtabns_bio_iov_iter_get_pages:
        .section "___ksymtab_gpl+bio_iov_iter_get_pages", "a"
__ksymtab_bio_iov_iter_get_pages:
        .long   bio_iov_iter_get_pages- .
        .long   __kstrtab_bio_iov_iter_get_pages- .
        .long   __kstrtabns_bio_iov_iter_get_pages- .
        .globl  bio_iov_iter_get_pages
        .type   bio_iov_iter_get_pages, @function
bio_iov_iter_get_pages:
        .type   bio_iov_iter_get_pages.cold, @function
bio_iov_iter_get_pages.cold:
        .size   bio_iov_iter_get_pages, .-bio_iov_iter_get_pages
        .size   bio_iov_iter_get_pages.cold, .-bio_iov_iter_get_pages.cold
        .type   __UNIQUE_ID___addressable_bio_iov_iter_get_pages499, @object
        .size   __UNIQUE_ID___addressable_bio_iov_iter_get_pages499, 8
__UNIQUE_ID___addressable_bio_iov_iter_get_pages499:
        .quad   bio_iov_iter_get_pages

So it does on mine too, but it doesn't guarantee all the platforms.