Questions tagged [mmap]
All about using memory mapped files. Questions on programming should be asked on Stack Overflow SE.
85 questions
2 votes
2 answers
176 views
Update mmap mapping to be readonly without overwriting existing data
I'm making a custom ELF loader to learn how the dynamic loader works behind the scenes, and one of the program headers often found in them is PT_GNU_RELRO, which tells the loader to make that segment ...
- 23
0 votes
1 answer
191 views
Writable and executable memory regions
I wrote a simple Python script to scan /proc/{pid}/maps for regions that are writable and executable on my computer. It came up with a few hits surprisingly, all private anonymous. Wondering why a ...
- 510
1 vote
0 answers
333 views
How can I pre-fault and lock memory pages that are mmap'd with MAP_PRIVATE?
I am writing a real-time linux application where I need to prevent any page faults from occuring after the initial startup of my application. My initial thought was just to call mlockall(MCL_CURRENT | ...
- 61
0 votes
1 answer
56 views
Is mmap holding a reference to the OFD specified by POSIX, or a Linux extension, and where is it documented?
I am using Open File Description (OFD) owned locks on Linux (fcntl with command F_OFD_SETLK). After locking a file, I memory mapped it, and closed the file descriptor. Another process tried to lock ...
- 35
6 votes
1 answer
2k views
Does mmap() update the page table after every page fault?
Based on my research on mmap(), I understand that mmap uses demand paging to copy in data to the kernel page cache only when the virtual memory address is touched, through page fault. If we are ...
- 63
2 votes
0 answers
294 views
Which consistency guarantees do POSIX shared memory objects have?
On Linux, POSIX shared memory objects [1] use a tmpfs via /dev/shm. A tmpfs in turn is said to "live completely in the page cache" [2] (I'm assuming swap has not been enabled). I am ...
3 votes
1 answer
140 views
How is 4kB RSS possible in Linux 4.x?
I have been the dev/maintainer of an open source IRC bot since the late 90s. The goal was always to make it as versatile & useful as possible in a small memory footprint. During the 2000s I also ...
- 131
4 votes
1 answer
1k views
Process memory layout - difference between heap, data and mmap areas
I see in the web many conflicting or unclear descriptions of the memory layout of a Linux process. Usually the common diagram looks like: And a common description would say that: The data segment ...
- 6,965
0 votes
0 answers
324 views
How to read watchdog registers on x86 Linux?
I want to read the Intel iTCO watchdog registers on my Intel Lynx Point system. I found the watchdog here: [ 5598.341020] iTCO_wdt iTCO_wdt.1.auto: Found a Lynx Point TCO device (Version=2, TCOBASE=...
- 153
0 votes
1 answer
165 views
Can mmap be used to create a file which references memory subset of another file?
I'm interested in writing a program that can create two files, second file would be a "view" of first file and if modified, the first file would also be modified. Is this possible to do with ...
0 votes
0 answers
108 views
Searching a 1T block device for a specific byte sequence at a specified offset
I'm performing data recovery after an accident with dd. In the longer term, I'll need to use some recovery tools to try and repair the file system In the meantime, there's an image on the system that ...
0 votes
1 answer
234 views
How to measure mmap I/O latency?
I have an application which appears to be slowing/blocking at the same time there's a lot of disk I/O going on, so I suspect it's I/O operations within the application which are blocking. I can't ...
- 73.9k
5 votes
1 answer
1k views
Linux HugeTLB: What is the advantage of the filesystem approach?
Moved Post Notice I just moved this question (with slight modifications) from a StackOverflow question (which I have deleted, since cross-posting is strongly discouraged), which has not been answered ...
- 231
0 votes
1 answer
386 views
Mapping Segment of Guest RAM to host file, in PPC QEMU
My desire is conceptually simple, I have a file (really a PCIe resource file from /sys/bus/pci/device/.... but that isn't too relevant) on the host that I want to make available somewhere in guest ...
- 411
1 vote
1 answer
331 views
Partial fsyncs when writing to block device
I'm writing my own data store directly on top of a block device. To ensure durability I want to sync to disk. But here's the thing: I want to sync only part of it. I'm keeping a journal for crash ...
