Re: [PATCH] mmap.2: document new MAP_FIXED_SAFE flag
From: Michal Hocko <mhocko@kernel.org>
Date: 2017-11-30 08:23:19
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On Wed 29-11-17 19:16:39, John Hubbard wrote: [...]
Hi Michal, I've taken the liberty of mostly rewriting this part, in order to more closely match the existing paragraphs; to fix minor typos; and to attempt to slightly clarify the paragraph. +.BR MAP_FIXED_SAFE " (since Linux 4.16)" +Similar to MAP_FIXED with respect to the +.I +addr +enforcement, but different in that MAP_FIXED_SAFE never clobbers a pre-existing +mapped range. If the requested range would collide with an existing +mapping, then this call fails with +.B EEXIST. +This flag can therefore be used as a way to atomically (with respect to other +threads) attempt to map an address range: one thread will succeed; all others +will report failure. Please note that older kernels which do not recognize this +flag will typically (upon detecting a collision with a pre-existing mapping) +fall back a "non-MAP_FIXED" type of behavior: they will return an address that +is different than the requested one. Therefore, backward-compatible software +should check the returned address against the requested address. +.TP
I have taken yours. Thanks a lot!
(I'm ignoring the naming, because there is another thread about that, so please just the above as "MAP_FIXED_whatever-is-chosen".)quoted
@@ -449,6 +461,12 @@ is not a valid file descriptor (and .B MAP_ANONYMOUS was not set). .TP +.B EEXIST +range covered by +.IR addr ,nit: trailing space on the above line.
fixed
quoted hunk ↗ jump to hunk
quoted
+.IR length +is clashing with an existing mapping. +.TP .B EINVAL We don't like .IR addr ,One other thing: reading through mmap.2, I now want to add this as well:diff --git a/man2/mmap.2 b/man2/mmap.2 index 622a7000d..780cad6d9 100644 --- a/man2/mmap.2 +++ b/man2/mmap.2@@ -222,20 +222,25 @@ part of the existing mapping(s) will be discarded. If the specified address cannot be used, .BR mmap () will fail. -Because requiring a fixed address for a mapping is less portable, -the use of this option is discouraged. +Software that aspires to be as portable as possible should use this option with +care, keeping in mind that different kernels and C libraries may set up quite +different mapping ranges....because that advice is just wrong (it presumes that "less portable" == "must be discouraged"). Should I send out a separate patch for that, or is it better to glom it together with this one?
yes please -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>