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From:   Kairui Song <kasong@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Dave Young <dyoung@redhat.com>, Baoquan He <bhe@redhat.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        kexec@lists.infradead.org, Kairui Song <kasong@redhat.com>
Subject: [RFC PATCH 1/3] vmcore: simplify read_from_olemem
Date:   Wed,  9 Sep 2020 15:50:14 +0800
Message-Id: <20200909075016.104407-2-kasong@redhat.com>
In-Reply-To: <20200909075016.104407-1-kasong@redhat.com>
References: <20200909075016.104407-1-kasong@redhat.com>
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Simplify the code logic, also helps reduce object size and stack usage.

Stack usage:
  Before: fs/proc/vmcore.c:106:9:read_from_oldmem.part.0  80     static
          fs/proc/vmcore.c:106:9:read_from_oldmem         16     static
  After:  fs/proc/vmcore.c:106:9:read_from_oldmem         80     static

Size of vmcore.o:
          text    data     bss     dec     hex filename
  Before: 7677     109      88    7874    1ec2 fs/proc/vmcore.o
  After:  7669     109      88    7866    1eba fs/proc/vmcore.o

Signed-off-by: Kairui Song <kasong@redhat.com>
---
 fs/proc/vmcore.c | 27 ++++++++++-----------------
 1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index c3a345c28a93..124c2066f3e5 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -108,25 +108,19 @@ ssize_t read_from_oldmem(char *buf, size_t count,
 			 bool encrypted)
 {
 	unsigned long pfn, offset;
-	size_t nr_bytes;
-	ssize_t read = 0, tmp;
+	size_t nr_bytes, to_copy = count;
+	ssize_t tmp;
 
-	if (!count)
-		return 0;
-
-	offset = (unsigned long)(*ppos % PAGE_SIZE);
+	offset = (unsigned long)(*ppos & (PAGE_SIZE - 1));
 	pfn = (unsigned long)(*ppos / PAGE_SIZE);
 
-	do {
-		if (count > (PAGE_SIZE - offset))
-			nr_bytes = PAGE_SIZE - offset;
-		else
-			nr_bytes = count;
+	while (to_copy) {
+		nr_bytes = min(to_copy, PAGE_SIZE - offset);
 
 		/* If pfn is not ram, return zeros for sparse dump files */
-		if (pfn_is_ram(pfn) == 0)
+		if (pfn_is_ram(pfn) == 0) {
 			memset(buf, 0, nr_bytes);
-		else {
+		} else {
 			if (encrypted)
 				tmp = copy_oldmem_page_encrypted(pfn, buf,
 								 nr_bytes,
@@ -140,14 +134,13 @@ ssize_t read_from_oldmem(char *buf, size_t count,
 				return tmp;
 		}
 		*ppos += nr_bytes;
-		count -= nr_bytes;
 		buf += nr_bytes;
-		read += nr_bytes;
+		to_copy -= nr_bytes;
 		++pfn;
 		offset = 0;
-	} while (count);
+	}
 
-	return read;
+	return count;
 }
 
 /*
-- 
2.26.2