On Thu, Aug 12, 2021 at 02:20:13PM +0100, Matthew Wilcox wrote:
> All the current xfstests for swap only test setting up a swap partition,
> not actually doing swap to it. That's why this bug went unnoticed for
> two years and why Steve French is still under the impression that
> swap-over-CIFS works (it doesn't). Could somebody adapt Dave's test
> into the xfstests framework?
Dave, can you just submit your test to xfstests?
> ----- Forwarded message from David Howells <dhowells@xxxxxxxxxx> -----
>
> Date: Thu, 12 Aug 2021 12:57:50 +0100
> From: David Howells <dhowells@xxxxxxxxxx>
> To: willy@xxxxxxxxxxxxx
> Cc: dhowells@xxxxxxxxxx, trond.myklebust@xxxxxxxxxxxxxxx,
> darrick.wong@xxxxxxxxxx, hch@xxxxxx, jlayton@xxxxxxxxxx,
> sfrench@xxxxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx,
> linux-nfs@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx,
> linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH 1/2] nfs: Fix write to swapfile failure due to
> generic_write_checks()
> User-Agent: StGit/0.23
>
> Trying to use a swapfile on NFS results in every DIO write failing with
> ETXTBSY because generic_write_checks(), as called by nfs_direct_write()
> from nfs_direct_IO(), forbids writes to swapfiles.
>
> Fix this by introducing a new kiocb flag, IOCB_SWAP, that's set by the swap
> code to indicate that the swapper is doing this operation and so overrule
> the check in generic_write_checks().
>
> Without this patch, the following is seen:
>
> Write error on dio swapfile (3800334336)
>
> Altering __swap_writepage() to show the error shows:
>
> Write error (-26) on dio swapfile (3800334336)
>
> Tested by swapping off all swap partitions and then swapping on a prepared
> NFS file (CONFIG_NFS_SWAP=y is also needed). Enough copies of the
> following program then need to be run to force swapping to occur (at least
> one per gigabyte of RAM):
>
> #include <stdbool.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/mman.h>
> int main()
> {
> unsigned int pid = getpid(), iterations = 0;
> size_t i, j, size = 1024 * 1024 * 1024;
> char *p;
> bool mismatch;
> p = malloc(size);
> if (!p) {
> perror("malloc");
> exit(1);
> }
> srand(pid);
> for (i = 0; i < size; i += 4)
> *(unsigned int *)(p + i) = rand();
> do {
> for (j = 0; j < 16; j++) {
> for (i = 0; i < size; i += 4096)
> *(unsigned int *)(p + i) += 1;
> iterations++;
> }
> mismatch = false;
> srand(pid);
> for (i = 0; i < size; i += 4) {
> unsigned int r = rand();
> unsigned int v = *(unsigned int *)(p + i);
> if (i % 4096 == 0)
> v -= iterations;
> if (v != r) {
> fprintf(stderr, "mismatch %zx: %x != %x (diff %x)\n",
> i, v, r, v - r);
> mismatch = true;
> }
> }
> } while (!mismatch);
> exit(1);
> }
>
> ----- End forwarded message -----
---end quoted text---
