On 12.07.21 11:58, Pankaj Gupta wrote:
MADV_POPULATE_READ and MADV_POPULATE_WRITE have been merged into upstream Linux via commit 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables"), part of v5.14-rc1. Let's document the behavior and error conditions of these new madvise() options. Cc: Alejandro Colomar <alx.manpages@xxxxxxxxx> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Oscar Salvador <osalvador@xxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxx> Cc: Linux API <linux-api@xxxxxxxxxxxxxxx> Cc: linux-mm@xxxxxxxxx Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> --- man2/madvise.2 | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 80 insertions(+) diff --git a/man2/madvise.2 b/man2/madvise.2 index f1f384c0c..3ec8c53a7 100644 --- a/man2/madvise.2 +++ b/man2/madvise.2 @@ -469,6 +469,59 @@ If a page is file-backed and dirty, it will be written back to the backing storage. The advice might be ignored for some pages in the range when it is not applicable. +.TP +.BR MADV_POPULATE_READ " (since Linux 5.14) +Populate (prefault) page tables readable for the whole range without actually +reading. Depending on the underlying mapping, map the shared zeropage, +preallocate memory or read the underlying file; files with holes might or +might not preallocate blocks. +Do not generate +.B SIGBUS +when populating fails, return an error instead. +.IP +If +.B MADV_POPULATE_READ +succeeds, all page tables have been populated (prefaulted) readable once. +If +.B MADV_POPULATE_READ +fails, some page tables might have been populated. +.IP +.B MADV_POPULATE_READ +cannot be applied to mappings without read permissions +and special mappings marked with the kernel-internal +.B VM_PFNMAP +and +.BR VM_IO . +.IP +Note that with +.BR MADV_POPULATE_READ , +the process can be killed at any moment when the system runs out of memory. +.TP +.BR MADV_POPULATE_WRITE " (since Linux 5.14) +Populate (prefault) page tables writable for the whole range without actually +writing. Depending on the underlying mapping, preallocate memory or read theIs this read or write? just reading and trying to understand :)
It's reading. Assume you have a file with existing content mapped into a process. Once you touch a page (read/write/execute) that maps to blocks with existing content, you'll have to load these blocks from disk first.
Thanks! :) -- Thanks, David / dhildenb
