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madvise.2: MADV_WIPEONFORK affects semantics

PATCH In review [email protected] 3 participants 4 messages Apr 28, 2020 – Apr 29, 2020 Man-pages: manual pages for Linux -- Sections 2, 3, 4, 5, and 7

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PATCH 1/1 madvise.2: MADV_WIPEONFORK affects semantics

Include MADV_WIPEONFORK in the list of advice values that have a semantic impact on the calling process.

---
 man2/madvise.2 | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
man2/madvise.2 +3 -1
index a0f3a477a..6c25f697a 100644
@@ -84,21 +84,23 @@ values have been added.
 The
 .I advice
 values listed below
 allow an application to tell the kernel how it expects to use
 some mapped or shared memory areas, so that the kernel can choose
 appropriate read-ahead and caching techniques.
 These
 .I advice
 values do not influence the semantics of the application
 (except in the case of
-.BR MADV_DONTNEED ),
+.BR MADV_DONTNEED
+and
+.BR MADV_WIPEONFORK ),
 but may influence its performance.
 All of the
 .I advice
 values listed here have analogs in the POSIX-specified
 .BR posix_madvise (3)
 function, and the values have the same meanings, with the exception of
 .BR MADV_DONTNEED .
 .PP
 The advice is indicated in the
 .I advice
Signature · 1 line
2.26.0
#1 lore raw

Hi Adam!

* Adam Langley <[email protected]>, 2020-04-28, 17:40:

Quoted 9 lines

These
.I advice
values do not influence the semantics of the application (except in the case of
-.BR MADV_DONTNEED ),
+.BR MADV_DONTNEED
+and
+.BR MADV_WIPEONFORK ),
but may influence its performance.

This sentence is in the "Conventional advice value" subsection, which defines only MADV_NORMAL, MADV_RANDOM, MADV_SEQUENTIAL, MADV_WILLNEED, and MADV_DONTNEED.

MADV_WIPEONFORK is defined in the "Linux-specific advice values" subsection, which already notes that "some of these operations change the semantics of memory accesses". (Perhaps s/some/most/ would be better wording.)

Signature · 1 line
Jakub Wilk
#2 lore raw

Hello Jakub (and Adam),

Quoted 14 lines · Jakub Wilk

Hi Adam!

* Adam Langley <[email protected]>, 2020-04-28, 17:40:

Quoted 9 lines

These
.I advice
values do not influence the semantics of the application (except in the case of
-.BR MADV_DONTNEED ),
+.BR MADV_DONTNEED
+and
+.BR MADV_WIPEONFORK ),
but may influence its performance.

Quoted 3 lines

This sentence is in the "Conventional advice value" subsection, which defines only MADV_NORMAL, MADV_RANDOM, MADV_SEQUENTIAL, MADV_WILLNEED, and MADV_DONTNEED.

(Good catch.)

Quoted 3 lines

MADV_WIPEONFORK is defined in the "Linux-specific advice values" subsection, which already notes that "some of these operations change the semantics of memory accesses".

<nod>

Quoted 2 lines

(Perhaps s/some/most/ would be better
wording.)

Perhaps you are right. Here's my take:

Do change memory semantics:
MADV_REMOVE
MADV_DONTFORK / MADV_DOFORK
MADV_HWPOISON
MADV_FREE
MADV_WIPEONFORK

Don't change memory semantics:
MADV_MERGEABLE / MADV_UNMERGEABLE
MADV_SOFT_OFFLINE
MADV_HUGEPAGE / MADV_NOHUGEPAGE
MADV_DONTDUMP / MADV_DODUMP

Do you agree?

"Some" is perhaps a little loose, but perhaps "most" is overstating it, so I'm not quite sure what to do here. Perhaps ad a line to each of the operations in the first category noting that the operations change memory semantics. THoughts?

Cheers,

Michael

Signature · 3 lines
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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#3 lore raw
In reply to Jakub Wilk
Quoted 8 lines · Jakub Wilk

This sentence is in the "Conventional advice value" subsection, which defines only MADV_NORMAL, MADV_RANDOM, MADV_SEQUENTIAL, MADV_WILLNEED, and MADV_DONTNEED.

MADV_WIPEONFORK is defined in the "Linux-specific advice values" subsection, which already notes that "some of these operations change the semantics of memory accesses". (Perhaps s/some/most/ would be better wording.)

Ah, thank you! Now that I look with that in mind, it makes sense.

AGL