133 lines
5.0 KiB
Diff
133 lines
5.0 KiB
Diff
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Date: Wed, 20 Sep 2006 10:56:08 +0200
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From: Alexandre Ratchov <alexandre.ratchov@bull.net>
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To: linux-ext4@vger.kernel.org
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Cc: Jean-Pierre Dion <jean-pierre.dion@bull.net>
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Subject: [patch] small fix for e2p_percent()
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hello,
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e2p_percent() doesn't work for zero percent (``mke2fs -m0'' gets killed with
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SIGFPE).
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is there a reason for not simply using 64 bit arithmetic? perhaps to avoid
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overflows for 7e+9TB file-systems :-), or just for correctness...
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So, here is the patch that fixes this. In order to avoid integer overflows,
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we pick 16 more bits to store ``base'' and do the exact division on 48 bits.
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Since ``100 * base'' always fits in 48 bits, there's never overflow. This
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still work if we remplace 'unsigned int' by 'unsigned long long'.
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cheers,
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-- Alexandre
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Signed-off-by: Alexandre Ratchov <alexandre.ratchov@bull.net>
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Index: e2fsprogs-1.39-my-patches-from-ted/lib/e2p/percent.c
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===================================================================
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--- e2fsprogs-1.39-my-patches-from-ted.orig/lib/e2p/percent.c
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+++ e2fsprogs-1.39-my-patches-from-ted/lib/e2p/percent.c
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@@ -14,18 +14,41 @@
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/*
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* We work really hard to calculate this accurately, while avoiding
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* an overflow. "Is there a hyphen in anal-retentive?" :-)
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+ *
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+ * -- "Yes there is, as in hair-splitting and nit-picking"
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*/
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unsigned int e2p_percent(int percent, unsigned int base)
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{
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- unsigned int mask = ~((1 << (sizeof(unsigned int) - 1) * 8) - 1);
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+ unsigned hi, lo, q, r;
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- if (100 % percent == 0)
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- return base / (100 / percent);
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- if (mask & base)
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- return (base / 100) * percent;
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- return base * percent / 100;
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+ /*
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+ * in order to avoid overflow we write 'base' as:
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+ *
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+ * base = hi * 2^16 + lo
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+ *
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+ * then we do all computations separately on 'hi' and 'lo'.
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+ * By using the definition of division:
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+ *
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+ * precent * base = result * 100 + reminder
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+ *
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+ * (reminder < 100), we obtain the exact value of 'result'
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+ * as follows:
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+ */
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+#define BITS 16
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+#define MASK ((1 << BITS) - 1)
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+
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+ hi = percent * (base >> BITS);
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+ lo = percent * (base & MASK);
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+
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+ q = ((hi / 100) << BITS) + lo / 100;
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+ r = ((hi % 100) << BITS) + lo % 100;
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+
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+ return q + r / 100;
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+#undef BITS
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+#undef MASK
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}
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+
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#ifdef DEBUG
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#include <unistd.h>
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#include <stdio.h>
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