Re: [PATCH v7] mm: shrink skip folio mapped by an exiting process

[David Hildenbrand]

On 10.07.24 06:02, Barry Song wrote:
> On Wed, Jul 10, 2024 at 3:59 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 10.07.24 05:32, Barry Song wrote:
> > > On Wed, Jul 10, 2024 at 9:23 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > > On Tue,  9 Jul 2024 20:31:15 +0800 Zhiguo Jiang <justinjiang@xxxxxxxx> wrote:
> > > >
> > > > > The releasing process of the non-shared anonymous folio mapped solely by
> > > > > an exiting process may go through two flows: 1) the anonymous folio is
> > > > > firstly is swaped-out into swapspace and transformed into a swp_entry
> > > > > in shrink_folio_list; 2) then the swp_entry is released in the process
> > > > > exiting flow. This will result in the high cpu load of releasing a
> > > > > non-shared anonymous folio mapped solely by an exiting process.
> > > > >
> > > > > When the low system memory and the exiting process exist at the same
> > > > > time, it will be likely to happen, because the non-shared anonymous
> > > > > folio mapped solely by an exiting process may be reclaimed by
> > > > > shrink_folio_list.
> > > > >
> > > > > This patch is that shrink skips the non-shared anonymous folio solely
> > > > > mapped by an exting process and this folio is only released directly in
> > > > > the process exiting flow, which will save swap-out time and alleviate
> > > > > the load of the process exiting.
> > > >
> > > > It would be helpful to provide some before-and-after runtime
> > > > measurements, please.  It's a performance optimization so please let's
> > > > see what effect it has.
> > >
> > > Hi Andrew,
> > >
> > > This was something I was curious about too, so I created a small test program
> > > that allocates and continuously writes to 256MB of memory. Using QEMU, I set
> > > up a small machine with only 300MB of RAM to trigger kswapd.
> > >
> > > qemu-system-aarch64 -M virt,gic-version=3,mte=off -nographic \
> > >    -smp cpus=4 -cpu max \
> > >    -m 300M -kernel arch/arm64/boot/Image
> > >
> > > The test program will be randomly terminated by its subprocess to trigger
> > > the use case of this patch.
> > >
> > > #include <stdio.h>
> > > #include <stdlib.h>
> > > #include <unistd.h>
> > > #include <string.h>
> > > #include <sys/types.h>
> > > #include <sys/wait.h>
> > > #include <time.h>
> > > #include <signal.h>
> > >
> > > #define MEMORY_SIZE (256 * 1024 * 1024)
> > >
> > > unsigned char *memory;
> > >
> > > void allocate_and_write_memory()
> > > {
> > >       memory = (unsigned char *)malloc(MEMORY_SIZE);
> > >       if (memory == NULL) {
> > >           perror("malloc");
> > >           exit(EXIT_FAILURE);
> > >       }
> > >
> > >       while (1)
> > >           memset(memory, 0x11, MEMORY_SIZE);
> > > }
> > >
> > > int main()
> > > {
> > >       pid_t pid;
> > >       srand(time(NULL));
> > >
> > >       pid = fork();
> > >
> > >       if (pid < 0) {
> > >           perror("fork");
> > >           exit(EXIT_FAILURE);
> > >       }
> > >
> > >       if (pid == 0) {
> > >           int delay = (rand() % 10000) + 10000;
> > >           usleep(delay * 1000);
> > >
> > >        /* kill parent when it is busy on swapping */
> > >           kill(getppid(), SIGKILL);
> > >           _exit(0);
> > >       } else {
> > >           allocate_and_write_memory();
> > >
> > >           wait(NULL);
> > >
> > >           free(memory);
> > >       }
> > >
> > >       return 0;
> > > }
> > >
> > > I tracked the number of folios that could be redundantly
> > > swapped out by adding a simple counter as shown below:
> > >
> > > @@ -879,6 +880,9 @@ static bool folio_referenced_one(struct folio *folio,
> > >                       check_stable_address_space(vma->vm_mm)) &&
> > >                       folio_test_swapbacked(folio) &&
> > >                       !folio_likely_mapped_shared(folio)) {
> > > +                       static long i, size;
> > > +                       size += folio_size(folio);
> > > +                       pr_err("index: %d skipped folio:%lx total size:%d\n", i++, (unsigned long)folio, size);
> > >                           pra->referenced = -1;
> > >                           page_vma_mapped_walk_done(&pvmw);
> > >                           return false;
> > >
> > >
> > > This is what I have observed:
> > >
> > > / # /home/barry/develop/linux/skip_swap_out_test
> > > [   82.925645] index: 0 skipped folio:fffffdffc0425400 total size:65536
> > > [   82.925960] index: 1 skipped folio:fffffdffc0425800 total size:131072
> > > [   82.927524] index: 2 skipped folio:fffffdffc0425c00 total size:196608
> > > [   82.928649] index: 3 skipped folio:fffffdffc0426000 total size:262144
> > > [   82.929383] index: 4 skipped folio:fffffdffc0426400 total size:327680
> > > [   82.929995] index: 5 skipped folio:fffffdffc0426800 total size:393216
> > > ...
> > > [   88.469130] index: 6112 skipped folio:fffffdffc0390080 total size:97230848
> > > [   88.469966] index: 6113 skipped folio:fffffdffc038d000 total size:97296384
> > > [   89.023414] index: 6114 skipped folio:fffffdffc0366cc0 total size:97300480
> > >
> > > I observed that this patch effectively skipped 6114 folios (either 4KB or 64KB
> > > mTHP), potentially reducing the swap-out by up to 92MB (97,300,480 bytes) during
> > > the process exit.
> > >
> > > Despite the numerous mistakes Zhiguo made in sending this patch, it is still
> > > quite valuable. Please consider pulling his v9 into the mm tree for testing.
> >
> > BTW, we dropped the folio_test_anon() check, but what about shmem? They
> > also do __folio_set_swapbacked()?
>
> my point is that the purpose is skipping redundant swap-out, if shmem is single
> mapped, they could be also skipped.

But they won't get necessarily *freed* when unmapping them. They might 
just continue living in tmpfs? where some other process might just map 
them later?

IMHO, there is a big difference here between anon and shmem. (well, 
anon_shmem would actually be different :) )

--
Cheers,

David / dhildenb