In preparation for discussing newly introduced mremap() behaviour to permit the move of multiple mappings at once, add a section to the mremap.2 man page to describe these operations in general. Signed-off-by: Lorenzo Stoakes --- man/man2/mremap.2 | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/man/man2/mremap.2 b/man/man2/mremap.2 index 2168ca728..4e3c8e54e 100644 --- a/man/man2/mremap.2 +++ b/man/man2/mremap.2 @@ -25,6 +25,20 @@ moving it at the same time (controlled by the argument and the available virtual address space). .P +Mappings can also simply be moved +(without any resizing) +by specifying equal +.I old_size +and +.I new_size +and using the +.B MREMAP_FIXED +flag +(see below). +The +.B MREMAP_DONTUNMAP +flag may also be specified. +.P .I old_address is the old address of the virtual memory block that you want to expand (or shrink). -- 2.50.1 Document the new behaviour introduced in Linux 6.17 whereby it is now possible to move multiple mappings in a single operation, as long as the operation is purely a move, that is old_size is equal to new_size and MREMAP_FIXED is specified. This change also explains the limitations of of this method and the possibility of partial failure. Finally, we pluralise language where it makes sense to so the documentation does not contradict either this new capability nor the pre-existing edge case. Example code is enclosed below demonstrating the behaviour which is now possible: int main(void) { unsigned long page_size = sysconf(_SC_PAGESIZE); void *ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0); void *tgt_ptr = mmap(NULL, 10 * page_size, PROT_NONE, MAP_ANON | MAP_PRIVATE, -1, 0); int i; if (ptr == MAP_FAILED || tgt_ptr == MAP_FAILED) { perror("mmap"); return EXIT_FAILURE; } /* Unmap every other page. */ for (i = 1; i < 10; i += 2) munmap(ptr + i * page_size, page_size); /* Now move all 5 distinct mappings to tgt_ptr. */ ptr = mremap(ptr, 10 * page_size, 10 * page_size, MREMAP_MAYMOVE | MREMAP_FIXED, tgt_ptr); if (ptr == MAP_FAILED) { perror("mremap"); return EXIT_FAILURE; } return EXIT_SUCCESS; } Signed-off-by: Lorenzo Stoakes --- man/man2/mremap.2 | 68 +++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 57 insertions(+), 11 deletions(-) diff --git a/man/man2/mremap.2 b/man/man2/mremap.2 index 4e3c8e54e..6d14bf627 100644 --- a/man/man2/mremap.2 +++ b/man/man2/mremap.2 @@ -35,22 +35,36 @@ and using the .B MREMAP_FIXED flag (see below). +Since Linux 6.17, +while +.I old_address +must be mapped, +.I old_size +may span multiple mappings +including unmapped areas between +them when performing a move like this. The .B MREMAP_DONTUNMAP flag may also be specified. .P +If the operation is not +simply moving mappings, +then +.I old_size +must span only a single mapping. +.P .I old_address -is the old address of the virtual memory block that you -want to expand (or shrink). +is the old address of the first virtual memory block that you +want to expand, shrink, and/or move. Note that .I old_address has to be page aligned. .I old_size -is the old size of the -virtual memory block. +is the size of the range containing +virtual memory blocks to be manipulated. .I new_size is the requested size of the -virtual memory block after the resize. +virtual memory blocks after the resize. An optional fifth argument, .IR new_address , may be provided; see the description of @@ -119,13 +133,42 @@ If is specified, then .B MREMAP_MAYMOVE must also be specified. +.IP +Since Linux 6.17, +if +.I old_size +is equal to +.I new_size +and +.B MREMAP_FIXED +is specified, then +.I old_size +may span beyond the mapping in which +.I old_address +resides. +In this case, +gaps between mappings in the original range +are maintained in the new range. +The whole operation is performed atomically +unless an error arises, +in which case the operation may be partially +completed, +that is, +some mappings may be moved and others not. +.IP +Moving multiple mappings is not permitted if +any of those mappings have either +been registered with +.BR userfaultfd (2) , +or map drivers that +specify their own custom address mapping logic. .TP .BR MREMAP_DONTUNMAP " (since Linux 5.7)" .\" commit e346b3813067d4b17383f975f197a9aa28a3b077 This flag, which must be used in conjunction with .BR MREMAP_MAYMOVE , -remaps a mapping to a new address but does not unmap the mapping at -.IR old_address . +remaps mappings to a new address but does not unmap them +from their original address. .IP The .B MREMAP_DONTUNMAP @@ -163,13 +206,13 @@ mapped. See NOTES for some possible applications of .BR MREMAP_DONTUNMAP . .P -If the memory segment specified by +If the memory segments specified by .I old_address and .I old_size -is locked (using +are locked (using .BR mlock (2) -or similar), then this lock is maintained when the segment is +or similar), then this lock is maintained when the segments are resized and/or relocated. As a consequence, the amount of memory locked by the process may change. .SH RETURN VALUE @@ -202,7 +245,10 @@ virtual memory address for this process. You can also get .B EFAULT even if there exist mappings that cover the -whole address space requested, but those mappings are of different types. +whole address space requested, but those mappings are of different types, +and the +.BR mremap () +operation being performed does not support this. .TP .B EINVAL An invalid argument was given. -- 2.50.1 There is pre-existing logic that appears to be undocumented for an mremap() shrink operation, where it turns out that the usual 'input range must span a single mapping' requirement no longer applies. In fact, it turns out that the input range specified by [old_address, old_address + old_size) may span any number of mappings. If shrinking in-place (that is, neither the MREMAP_FIXED nor MREMAP_DONTUNMAP flags are specified), then the new span may also span any number of VMAs - [old_address, old_address + new_size). If shrinking and moving, the range specified by [old_address, old_address + new_size) must span a single VMA. There must be at least one VMA contained within the [old_address, old_address + old_size) range, and old_address must be within the range of a VMA. Explicitly document this. Signed-off-by: Lorenzo Stoakes --- man/man2/mremap.2 | 31 +++++++++++++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-) diff --git a/man/man2/mremap.2 b/man/man2/mremap.2 index 6d14bf627..53d4eda29 100644 --- a/man/man2/mremap.2 +++ b/man/man2/mremap.2 @@ -47,8 +47,35 @@ The .B MREMAP_DONTUNMAP flag may also be specified. .P -If the operation is not -simply moving mappings, +Equally, if the operation performs a shrink, +that is if +.I old_size +is greater than +.IR new_size , +then +.I old_size +may also span multiple mappings +which do not have to be +adjacent to one another. +If this shrink is performed +in-place, +that is, +neither +.BR MREMAP_FIXED , +nor +.B MREMAP_DONTUNMAP +are specified, +.I new_size +may also span multiple VMAs. +However, if the range is moved, +then +.I new_size +must span only a single mapping. +.P +If the operation is neither a +.B MREMAP_FIXED +move +nor a shrink, then .I old_size must span only a single mapping. -- 2.50.1