Re: [PATCH] makedumpfile: readpage_elf: handle 0-pages not stored in the ELF file

[Petr Tesarik <ptesarik@suse.cz>]

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Petr T

On Wed, 27 Jan 2016 08:58:21 +0100
Petr Tesarik <ptesarik@suse.cz> wrote:

> Hello Atsushi-san,
> 
> On Wed, 27 Jan 2016 02:21:57 +0000
> Atsushi Kumagai <ats-kumagai@wm.jp.nec.com> wrote:
> 
> > Hello Ivan,
> > 
> > >Handle pages filled with zeros that are not stored in the ELF file
> > >directly, but have addresses between p_filesz and p_memsz of a segment.
> > 
> > This fix looks reasonable, thanks for your work.
> > 
> > >This allows makedumpfile to dump-dmesg from a previously makedumpfile-ed
> > >vmcore where all 0-pages were excluded (dump_level & 1) for example.
> > 
> > I have some comments below:
> 
> Actually, I have already hit another issue with filtered ELF files.
> When refiltering an ELF file that had already been filtered, the memory
> size vs. file size can cause wrong max_mapnr calculation for a
> compressed file. In turn, if some pages were removed at the end of
> physical memory, those pages are not marked as filtered in the output
> file, but instead the total RAM appears to be smaller by that amount.
> 
> I don't think my patch addresses the same issue that Ivan found, but it
> definitely touches the same area. Please have a look.
> 
> Petr Tesarik

From: Petr Tesarik <ptesarik@suse.cz>
Date: Thu Aug 20 10:43:22 2015 +0200
Subject: Keep segment memory size when re-filtering ELF dumps

Originally, makedumpfile was designed to read from /proc/vmcore, where
each segment's p_memsz is equal to its p_filesz. However, makedumpfile
can also be used to re-filter an already filtered ELF dump file, where
memory size may be larger than file size. In that case the memory size
should be used as the size of the segment. This affects:

1. max_mapnr
   This value is computed as the highest phys_end. If the last segment
   was filtered, max_mapnr may be too small, and the crash utility will
   refuse to read that memory (even with --zero_excluded).

2. p_memsz field in ELF dumps
   The resulting ELF segment p_memsz will be capped to original file's
   p_filesz, ignoring the original p_memsz.

3. memory holes in KDUMP dumps
   Pages excluded in the original ELF dump will be appear as memory
   holes in the resulting KDUMP file's first bitmap.

4. vtop translation
   Virtual addresses that were filtered out in the original ELF file
   cannot be translated to physical addresses using the generic vtop
   translation.

My fix uses p_memsz to set physical and virtual extents of ELF segments,
because this is the actual size. However, file size is important when
accessing page data, so it must be stored separately and checked where
appropriate. Last but not least, pages that were filtered in the original
dump file must also be filtered in the destination file, i.e. pages
between p_filesz and p_memsz must have their corresponding bit cleared in
the 2nd bitmap.

Signed-off-by: Petr Tesarik <ptesarik@suse.com>

---
 elf_info.c     |   37 ++++++++++++++++++++++++++++++-------
 elf_info.h     |    4 ++++
 makedumpfile.c |   26 ++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 7 deletions(-)

--- a/elf_info.c
+++ b/elf_info.c
@@ -38,6 +38,7 @@
 
 struct pt_load_segment {
 	off_t			file_offset;
+	off_t			file_size;
 	unsigned long long	phys_start;
 	unsigned long long	phys_end;
 	unsigned long long	virt_start;
@@ -162,10 +163,11 @@ dump_Elf_load(Elf64_Phdr *prog, int num_
 
 	pls = &pt_loads[num_load];
 	pls->phys_start  = prog->p_paddr;
-	pls->phys_end    = pls->phys_start + prog->p_filesz;
+	pls->phys_end    = pls->phys_start + prog->p_memsz;
 	pls->virt_start  = prog->p_vaddr;
-	pls->virt_end    = pls->virt_start + prog->p_filesz;
+	pls->virt_end    = pls->virt_start + prog->p_memsz;
 	pls->file_offset = prog->p_offset;
+	pls->file_size   = prog->p_filesz;
 
 	DEBUG_MSG("LOAD (%d)\n", num_load);
 	DEBUG_MSG("  phys_start : %llx\n", pls->phys_start);
@@ -462,7 +464,7 @@ paddr_to_offset(unsigned long long paddr
 	for (i = offset = 0; i < num_pt_loads; i++) {
 		pls = &pt_loads[i];
 		if ((paddr >= pls->phys_start)
-		    && (paddr < pls->phys_end)) {
+		    && (paddr < pls->phys_start + pls->file_size)) {
 			offset = (off_t)(paddr - pls->phys_start) +
 				pls->file_offset;
 			break;
@@ -480,16 +482,14 @@ paddr_to_offset2(unsigned long long padd
 {
 	int i;
 	off_t offset;
-	unsigned long long len;
 	struct pt_load_segment *pls;
 
 	for (i = offset = 0; i < num_pt_loads; i++) {
 		pls = &pt_loads[i];
-		len = pls->phys_end - pls->phys_start;
 		if ((paddr >= pls->phys_start)
-		    && (paddr < pls->phys_end)
+		    && (paddr < pls->phys_start + pls->file_size)
 		    && (hint >= pls->file_offset)
-		    && (hint < pls->file_offset + len)) {
+		    && (hint < pls->file_offset + pls->file_size)) {
 			offset = (off_t)(paddr - pls->phys_start) +
 				pls->file_offset;
 			break;
@@ -1095,6 +1095,29 @@ get_pt_load(int idx,
 
 	return TRUE;
 }
+
+int
+get_pt_extents(int idx,
+	unsigned long long *phys_start,
+	unsigned long long *phys_end,
+	off_t *file_size)
+{
+	struct pt_load_segment *pls;
+
+	if (num_pt_loads <= idx)
+		return FALSE;
+
+	pls = &pt_loads[idx];
+
+	if (phys_start)
+		*phys_start = pls->phys_start;
+	if (phys_end)
+		*phys_end   = pls->phys_end;
+	if (file_size)
+		*file_size = pls->file_size;
+
+	return TRUE;
+}
 
 unsigned int
 get_num_pt_loads(void)
--- a/elf_info.h
+++ b/elf_info.h
@@ -61,6 +61,10 @@ int get_pt_load(int idx,
 	unsigned long long *phys_end,
 	unsigned long long *virt_start,
 	unsigned long long *virt_end);
+int get_pt_extents(int idx,
+	unsigned long long *phys_start,
+	unsigned long long *phys_end,
+	off_t *file_size);
 unsigned int get_num_pt_loads(void);
 
 void set_nr_cpus(int num);
--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -4395,6 +4395,26 @@ read_start_flat_header(void)
 	return TRUE;
 }
 
+static void
+exclude_nodata_pages(void)
+{
+	int i;
+	unsigned long long phys_start, phys_end;
+	off_t file_size;
+
+	i = 0;
+	while (get_pt_extents(i, &phys_start, &phys_end, &file_size)) {
+		unsigned long long pfn, pfn_end;
+
+		pfn = paddr_to_pfn(phys_start + file_size);
+		pfn_end = paddr_to_pfn(phys_end);
+		while (pfn <= pfn_end)
+			clear_bit_on_2nd_bitmap(pfn);
+			++pfn;
+		++i;
+	}
+}
+
 int
 read_flat_data_header(struct makedumpfile_data_header *fdh)
 {
@@ -6087,6 +6107,12 @@ create_2nd_bitmap(struct cycle *cycle)
 	}
 
 	/*
+	 * If re-filtering ELF dump, exclude pages that were already
+	 * excluded in the original file.
+	 */
+	exclude_nodata_pages();
+
+	/*
 	 * Exclude cache pages, cache private pages, user data pages,
 	 * and hwpoison pages.
 	 */

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