Re: [GIT PULL] Btrfs updates for 5.16

[Qu Wenruo]

On 2021/11/2 04:03, Linus Torvalds wrote:
> On Mon, Nov 1, 2021 at 9:46 AM David Sterba <dsterba@xxxxxxxx> wrote:
> > There's a merge conflict due to the last minute 5.15 changes (kmap
> > reverts) and the conflict is not trivial.
>
> You don't say.
>
> I ended up just re-doing that resolution entirely, and as I did so, I
> think I found a bug in the original revert that caused the conflict in
> the first place.
>
> And since that revert made it into 5.15, I felt like I had to fix that
> bug first - and separately - so that the fix can be backported to
> stable.
>
> I then re-did my merge on top of that hopefully fixed state, and maybe
> it's correct.
>
> Or maybe I messed up entirely.
>
> I did end up comparing it to your other branch too, but that was
> equally as messy, apart from the "ok, I can mindlessly just take your
> side".
>
> And it was fairly different from what I had done in my merge
> resolution, so who knows.
>
> ANYWAY. What I'm trying to say is that you should look very very
> carefully at commits
>
>    2cf3f8133bda ("btrfs: fix lzo_decompress_bio() kmap leakage")

Since I'm doing the revert manually for lzo part, I double checked the code.

It turns out, your fix is the same as the original version I sent to
David (although not through the mail list).
Full patch attached.

@@ -345,8 +358,9 @@ int lzo_decompress_bio(struct list_head *ws, struct
compressed_bio *cb)
 		       (cur_in + LZO_LEN - 1) / sectorsize);
 		cur_page = cb->compressed_pages[cur_in / PAGE_SIZE];
 		ASSERT(cur_page);
-		seg_len = read_compress_length(page_address(cur_page) +
-					       offset_in_page(cur_in));
+		kaddr = kmap(cur_page);
+		seg_len = read_compress_length(kaddr + offset_in_page(cur_in));
+		kunmap(cur_page);
 		cur_in += LZO_LEN;

Thus it looks like by somehow my version is not applied?

Thanks,
Qu

>    037c50bfbeb3 ("Merge tag 'for-5.16-tag' of git://git.../linux") >
> because I marked that first one for stable, and the second is
> obviously my entirely untested merge.
>
> It makes sense to me, but apart from "it builds", I've not actually
> tested any of it. This is all purely from looking at the code and
> trying to figure out what the RightThing(tm) is.
>
> Obviously the kmap thing tends to only be noticeable on 32-bit
> platforms, and that lzo_decompress_bio() bug also needs just the
> proper filesystem settings to trigger in the first place.
>
> Again - please take a careful look. Both at my merge and at that
> alleged kmap fix.
>
>                            Linus
From 38c48395c1f3cb8aaea449dbaa81cf0efdc012d8 Mon Sep 17 00:00:00 2001
From: Qu Wenruo <wqu@xxxxxxxx>
Date: Wed, 27 Oct 2021 17:05:36 +0800
Subject: [PATCH] Revert "btrfs: compression: drop kmap/kunmap from lzo"

This reverts commit 8c945d32e60427cbc0859cf7045bbe6196bb03d8.
---
 fs/btrfs/lzo.c | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/lzo.c b/fs/btrfs/lzo.c
index c25dfd1a8a54..295bbc13ace6 100644
--- a/fs/btrfs/lzo.c
+++ b/fs/btrfs/lzo.c
@@ -141,7 +141,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 	*total_in = 0;
 
 	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
-	data_in = page_address(in_page);
+	data_in = kmap(in_page);
 
 	/*
 	 * store the size of all chunks of compressed data in
@@ -152,7 +152,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 		ret = -ENOMEM;
 		goto out;
 	}
-	cpage_out = page_address(out_page);
+	cpage_out = kmap(out_page);
 	out_offset = LZO_LEN;
 	tot_out = LZO_LEN;
 	pages[0] = out_page;
@@ -210,6 +210,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 				if (out_len == 0 && tot_in >= len)
 					break;
 
+				kunmap(out_page);
 				if (nr_pages == nr_dest_pages) {
 					out_page = NULL;
 					ret = -E2BIG;
@@ -221,7 +222,7 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 					ret = -ENOMEM;
 					goto out;
 				}
-				cpage_out = page_address(out_page);
+				cpage_out = kmap(out_page);
 				pages[nr_pages++] = out_page;
 
 				pg_bytes_left = PAGE_SIZE;
@@ -243,11 +244,12 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 			break;
 
 		bytes_left = len - tot_in;
+		kunmap(in_page);
 		put_page(in_page);
 
 		start += PAGE_SIZE;
 		in_page = find_get_page(mapping, start >> PAGE_SHIFT);
-		data_in = page_address(in_page);
+		data_in = kmap(in_page);
 		in_len = min(bytes_left, PAGE_SIZE);
 	}
 
@@ -257,17 +259,22 @@ int lzo_compress_pages(struct list_head *ws, struct address_space *mapping,
 	}
 
 	/* store the size of all chunks of compressed data */
-	sizes_ptr = page_address(pages[0]);
+	sizes_ptr = kmap_local_page(pages[0]);
 	write_compress_length(sizes_ptr, tot_out);
+	kunmap_local(sizes_ptr);
 
 	ret = 0;
 	*total_out = tot_out;
 	*total_in = tot_in;
 out:
 	*out_pages = nr_pages;
+	if (out_page)
+		kunmap(out_page);
 
-	if (in_page)
+	if (in_page) {
+		kunmap(in_page);
 		put_page(in_page);
+	}
 
 	return ret;
 }
@@ -283,6 +290,7 @@ static void copy_compressed_segment(struct compressed_bio *cb,
 	u32 orig_in = *cur_in;
 
 	while (*cur_in < orig_in + len) {
+		char *kaddr;
 		struct page *cur_page;
 		u32 copy_len = min_t(u32, PAGE_SIZE - offset_in_page(*cur_in),
 					  orig_in + len - *cur_in);
@@ -290,9 +298,11 @@ static void copy_compressed_segment(struct compressed_bio *cb,
 		ASSERT(copy_len);
 		cur_page = cb->compressed_pages[*cur_in / PAGE_SIZE];
 
+		kaddr = kmap(cur_page);
 		memcpy(dest + *cur_in - orig_in,
-			page_address(cur_page) + offset_in_page(*cur_in),
+			kaddr + offset_in_page(*cur_in),
 			copy_len);
+		kunmap(cur_page);
 
 		*cur_in += copy_len;
 	}
@@ -303,6 +313,7 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 	struct workspace *workspace = list_entry(ws, struct workspace, list);
 	const struct btrfs_fs_info *fs_info = btrfs_sb(cb->inode->i_sb);
 	const u32 sectorsize = fs_info->sectorsize;
+	char *kaddr;
 	int ret;
 	/* Compressed data length, can be unaligned */
 	u32 len_in;
@@ -311,7 +322,9 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 	/* Bytes decompressed so far */
 	u32 cur_out = 0;
 
-	len_in = read_compress_length(page_address(cb->compressed_pages[0]));
+	kaddr = kmap(cb->compressed_pages[0]);
+	len_in = read_compress_length(kaddr);
+	kunmap(cb->compressed_pages[0]);
 	cur_in += LZO_LEN;
 
 	/*
@@ -345,8 +358,9 @@ int lzo_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
 		       (cur_in + LZO_LEN - 1) / sectorsize);
 		cur_page = cb->compressed_pages[cur_in / PAGE_SIZE];
 		ASSERT(cur_page);
-		seg_len = read_compress_length(page_address(cur_page) +
-					       offset_in_page(cur_in));
+		kaddr = kmap(cur_page);
+		seg_len = read_compress_length(kaddr + offset_in_page(cur_in));
+		kunmap(cur_page);
 		cur_in += LZO_LEN;
 
 		/* Copy the compressed segment payload into workspace */
@@ -431,7 +445,7 @@ int lzo_decompress(struct list_head *ws, unsigned char *data_in,
 	destlen = min_t(unsigned long, destlen, PAGE_SIZE);
 	bytes = min_t(unsigned long, destlen, out_len - start_byte);
 
-	kaddr = page_address(dest_page);
+	kaddr = kmap_local_page(dest_page);
 	memcpy(kaddr, workspace->buf + start_byte, bytes);
 
 	/*
@@ -441,6 +455,7 @@ int lzo_decompress(struct list_head *ws, unsigned char *data_in,
 	 */
 	if (bytes < destlen)
 		memset(kaddr+bytes, 0, destlen-bytes);
+	kunmap_local(kaddr);
 out:
 	return ret;
 }
-- 
2.33.1