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d6ff4c8f65Dumping processes with large allocated and mostly not-faulted areas is
very slow.
Borrowing a test case from Tavian Barnes:
int main(void) {
char *mem = mmap(NULL, 1ULL << 40, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_NORESERVE | MAP_PRIVATE, -1, 0);
printf("%p %m\n", mem);
if (mem != MAP_FAILED) {
mem[0] = 1;
}
abort();
}
That's 1TB of almost completely not-populated area.
On my test box it takes 13-14 seconds to dump.
The profile shows:
- 99.89% 0.00% a.out
entry_SYSCALL_64_after_hwframe
do_syscall_64
syscall_exit_to_user_mode
arch_do_signal_or_restart
- get_signal
- 99.89% do_coredump
- 99.88% elf_core_dump
- dump_user_range
- 98.12% get_dump_page
- 64.19% __get_user_pages
- 40.92% gup_vma_lookup
- find_vma
- mt_find
4.21% __rcu_read_lock
1.33% __rcu_read_unlock
- 3.14% check_vma_flags
0.68% vma_is_secretmem
0.61% __cond_resched
0.60% vma_pgtable_walk_end
0.59% vma_pgtable_walk_begin
0.58% no_page_table
- 15.13% down_read_killable
0.69% __cond_resched
13.84% up_read
0.58% __cond_resched
Almost 29% of the time is spent relocking the mmap semaphore between
calls to get_dump_page() which find nothing.
Whacking that results in times of 10 seconds (down from 13-14).
While here make the thing killable.
The real problem is the page-sized iteration and the real fix would
patch it up instead. It is left as an exercise for the mm-familiar
reader.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250119103205.2172432-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.