Re: [Qemu-devel] [RFC 2/2] block/file-posix: verify page cache is not used

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> This commit is for debugging only.  Do not merge it.
> 
> mincore(2) checks whether pages are resident.  Use it to verify that
> page cache has been dropped.
> 
> You can trigger a verification failure by mmapping the image file from
> another process and loading a byte from a page so that it becomes
> resident.  bdrv_co_invalidate_cache() will fail while the process is
> alive.

It doesn't seem a bad diagnostic to keep in (with a switch to activate)
for when we're faced with some weird corruption on some weird storage
system.

Dave

> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  block/file-posix.c | 71 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 71 insertions(+)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index df4f52919f..d3105269c6 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2236,6 +2236,75 @@ static int coroutine_fn 
> raw_co_block_status(BlockDriverState *bs,
>      return ret | BDRV_BLOCK_OFFSET_VALID;
>  }
>  
> +static bool is_mincore(void *addr, size_t length)
> +{
> +    size_t vec_len = DIV_ROUND_UP(length, sysconf(_SC_PAGESIZE));
> +    unsigned char *vec;
> +    size_t i;
> +    int ret;
> +    bool incore = false;
> +
> +    vec = g_malloc(vec_len);
> +    ret = mincore(addr, length, vec);
> +    if (ret < 0) {
> +        incore = true;
> +        goto out;
> +    }
> +
> +    for (i = 0; i < vec_len; i++) {
> +        if (vec[i] & 0x1) {
> +            incore = true;
> +            break;
> +        }
> +    }
> +
> +out:
> +    g_free(vec);
> +    return incore;
> +}
> +
> +static void check_not_in_page_cache(BlockDriverState *bs, Error **errp)
> +{
> +    const size_t WINDOW_SIZE = 128 * 1024 * 1024;
> +    BDRVRawState *s = bs->opaque;
> +    void *window = NULL;
> +    size_t length = 0;
> +    off_t end;
> +    off_t offset;
> +
> +    end = raw_getlength(bs);
> +
> +    for (offset = 0; offset < end; offset += WINDOW_SIZE) {
> +        void *new_window;
> +        size_t new_length = MIN(end - offset, WINDOW_SIZE);
> +
> +        if (new_length != length) {
> +            munmap(window, length);
> +            window = NULL;
> +            length = 0;
> +        }
> +
> +        new_window = mmap(window, new_length, PROT_NONE, MAP_PRIVATE,
> +                          s->fd, offset);
> +        if (new_window == MAP_FAILED) {
> +            error_setg_errno(errp, errno, "mmap failed");
> +            break;
> +        }
> +
> +        window = new_window;
> +        length = new_length;
> +
> +        if (is_mincore(window, length)) {
> +            error_setg(errp, "page cache still in use!");
> +            break;
> +        }
> +    }
> +
> +    if (window) {
> +        munmap(window, length);
> +    }
> +}
> +
>  static void coroutine_fn raw_co_invalidate_cache(BlockDriverState *bs,
>                                                   Error **errp)
>  {
> @@ -2270,6 +2339,8 @@ static void coroutine_fn 
> raw_co_invalidate_cache(BlockDriverState *bs,
>          return;
>      }
>  #endif /* __linux__ */
> +
> +    check_not_in_page_cache(bs, errp);
>  }
>  
>  static coroutine_fn BlockAIOCB *raw_aio_pdiscard(BlockDriverState *bs,
> -- 
> 2.14.3
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK