Re: [Libguestfs] [PATCH libnbd v3 2/3] copy: Preserve the host page cache when reading from local files.
On Thu, Feb 25, 2021 at 03:25:48PM -0600, Eric Blake wrote:
On 2/25/21 11:34 AM, Richard W.M. Jones wrote:
> +/* If we are going to attempt page cache mapping which tries not to
> + * disturb the page cache when reading a file. Only do this on Linux
Works, but took me a couple reads to decide it wasn't a sentence
fragment. Would it be any better as s/If/Whether/?
I'll see if I can clarify this before I push it.
> +static void page_size_init (void)
__attribute__((constructor));
> +static void
> +page_size_init (void)
> +{
> + page_size = sysconf (_SC_PAGE_SIZE);
Technically, sysconf() can fail; but if it fails on _SC_PAGE_SIZE,
you've got bigger problems ;)
Maybe best to assert if it fails?
> + /* Only bother with whole pages. */
> + offset = ROUND_UP (orig_offset, page_size);
> + len = orig_len - (offset - orig_offset);
> + len = ROUND_DOWN (len, page_size);
What assurance do we have that the rest of nbdcopy is favoring a stride
that is a multiple of page_size, so that we aren't stranding partial
pages everywhere?
It's not an assurance, but in general nbdcopy will favour blocks
aligned to very large boundaries (32M). From the level of
copy/file-ops.c we can't really be sure of this and so must assume
arbitrary alignment.
Looks good.
Thanks for the reviews.
Rich.
--
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