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Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum

From: Max Reitz
Subject: Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum
Date: Mon, 12 Jul 2021 09:47:05 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 28.06.21 21:10, Eric Blake wrote:
+++ b/include/block/block_int.h
@@ -347,6 +347,11 @@ struct BlockDriver {
        * clamped to bdrv_getlength() and aligned to request_alignment,
        * as well as non-NULL pnum, map, and file; in turn, the driver
        * must return an error or set pnum to an aligned non-zero value.
+     *
+     * Note that @bytes is just a hint on how big of a region the
+     * caller wants to inspect.  It is not a limit on *pnum.
+     * Implementations are free to return larger values of *pnum if
+     * doing so does not incur a performance penalty.
Worth mention that the cache will benefit of it?
Oh, right, absolutely.  Like so:

"block/io.c's bdrv_co_block_status() will clamp *pnum before returning it to
its caller, but it itself can still make use of the unclamped *pnum value.
Specifically, the block-status cache for protocol nodes will benefit from
storing as large a region as possible."
How about this tweak to the wording to make it flow a little better:

block/io.c's bdrv_co_block_status() will utilize an unclamped *pnum
value for the block-status cache on protocol nodes, prior to clamping
*pnum for return to its caller.

Sure, thanks!


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