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Why not just use the SHA1 only?Why not just use the SHA1 only?Posted Nov 18, 2008 4:30 UTC (Tue) by nevets (subscriber, #11875)In reply to: Why not just use the SHA1 only? by nix Parent article: /dev/ksm: dynamic memory sharing From your friends at Wikipedia, there is an article on SHA-1 SHA-1 (as well as SHA-0) produces a 160-bit digest from a message with a maximum length of (2^64 − 1) bits This looks like it can be much bigger than 128 bytes.
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Why not just use the SHA1 only? Posted Nov 18, 2008 16:01 UTC (Tue) by nix (subscriber, #2304) [Link]
Yeah, but for speed reasons they're only hashing the first N bytes (I
think it's 128), rather than the whole page. It's a sensible tradeoff, I think.
Why not just use the SHA1 only? Posted May 23, 2010 9:47 UTC (Sun) by rafal.maj (guest, #66508) [Link]
This doesn't make sense, they are for sure hashing entire page.
Why not just use the SHA1 only? Posted May 23, 2010 10:57 UTC (Sun) by johill (subscriber, #25196) [Link] Well, but you clearly didn't read the code:
+#define PAGECMP_OFFSET 128
+#define PAGEHASH_SIZE (PAGECMP_OFFSET ? PAGECMP_OFFSET : PAGE_SIZE)
+/* hash the page */
+static void page_hash(struct page *page, unsigned char *digest)
+{
+ struct scatterlist sg;
+ struct hash_desc desc;
+
+ sg_init_table(&sg, 1);
+ sg_set_page(&sg, page, PAGEHASH_SIZE, 0);
+ desc.tfm = tfm;
+ desc.flags = 0;
+ crypto_hash_digest(&desc, &sg, PAGEHASH_SIZE, digest);
+}
and it does "for sure" make sense since it's just a way to speed up matching, and just using 128 bytes means much less loading from memory. Tuning the 128 might make sense, potentially.
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