Always add MAP_NORESERVE flag in mmap for a regular file?

Question

According to mmap 's manual :

MAP_NORESERVE

Do not reserve swap space for this mapping. When swap space is reserved, one has the guarantee that it is possible to modify the mapping. When swap space is not reserved one might get SIGSEGV upon a write if no physical memory is available.

To my understanding, if a regular file is mapped into the virtual address range, there is no need for any swap space. Only MAP_ANONYMOUS may need some swap space.

So, is it correct to always add MAP_NORESERVE flag in mmap for a regular file?

Update:

To be more specific, is it correct to always add MAP_NORESERVE flag in mmap for a regular file, when MAP_SHARED is used?

Answer 1

To my understanding, if a regular file is mapped into the virtual address range, there is no need for any swap space. Only MAP_ANONYMOUS may need some swap space.

That depends on the mmap flags. If a regular file is mapped with MAP_PRIVATE then the memory region is initialized from the file but not backed by the file. The system will need swap space for such a mapping if it decides to swap out any of its pages.

So, is it correct to always add MAP_NORESERVE flag in mmap for a regular file?

It is not incorrect to specify MAP_NORESERVE for any mapping. It's simply a question of what guarantees you want to have about program behavior.

Moreover, you seem looking at this from the wrong direction. If a particular mapping can never require swap space then the system will not reserve swap space for it, regardless of the flags. It doesn't hurt to use MAP_NORESERVE in such a case, but it doesn't help, either, so what would be the point?

On the other hand, if you want to be sure that mapping cannot fail on account of using MAP_NORESERVE then the most appropriate course of action is to avoid using that flag. You can completely ignore its existence if you wish, and in fact you should do so if you want maximum portability, because MAP_NORESERVE is a Linux extension not specified by POSIX.

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Update:

As I understand it, you are asserting that you can reproducibly observe successful mapping of existing ranges of existing files with MAP_SHARED to require MAP_NORESERVE . That is, two such mapping attempts that differ only in whether MAP_NORESERVE is specified will produce different results for you, in a manner that you can predict and reliably reproduce.

I find that surprising, even dubious. I do not expect pages of a process's virtual address space that the page table maps to existing regions of a regular file to have any association with swap space, and therefore I do not expect the system to try to reserve any swap space for such pages when it establishes a mapping, flags notwithstanding. If you genuinely observe different behavior then I would attribute it to a library or kernel bug, about which you should file an issue.

Consider, for example, the GNU libc manual, which explicitly says that a memory mapping can be larger than physical memory and swap space , but does not document Linux-specific MAP_NORESERVE at all.

With that said, again, it is not incorrect (on Linux) to specify MAP_NORESERVE for any given memory mapping. I expect it to be meaningless for a MAP_SHARED mapping of an existing region of a regular file, however, so I would not consider it good -- much less best -- practice to routinely use that flag for such mappings. On the other hand, if specifying that flag works around a library or kernel bug that otherwise interferes with establishing certain mappings then I see no particular reason to avoid doing that, but I would expect each such use to be accompanied by a documentary comment explaining why that flag is used.