78 lines
2.8 KiB
Text
78 lines
2.8 KiB
Text
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pagemap, from the userspace perspective
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---------------------------------------
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pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
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userspace programs to examine the page tables and related information by
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reading files in /proc.
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There are three components to pagemap:
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* /proc/pid/pagemap. This file lets a userspace process find out which
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physical frame each virtual page is mapped to. It contains one 64-bit
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value for each virtual page, containing the following data (from
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fs/proc/task_mmu.c, above pagemap_read):
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* Bits 0-55 page frame number (PFN) if present
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* Bits 0-4 swap type if swapped
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* Bits 5-55 swap offset if swapped
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* Bits 55-60 page shift (page size = 1<<page shift)
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* Bit 61 reserved for future use
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* Bit 62 page swapped
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* Bit 63 page present
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If the page is not present but in swap, then the PFN contains an
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encoding of the swap file number and the page's offset into the
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swap. Unmapped pages return a null PFN. This allows determining
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precisely which pages are mapped (or in swap) and comparing mapped
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pages between processes.
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Efficient users of this interface will use /proc/pid/maps to
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determine which areas of memory are actually mapped and llseek to
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skip over unmapped regions.
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* /proc/kpagecount. This file contains a 64-bit count of the number of
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times each page is mapped, indexed by PFN.
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* /proc/kpageflags. This file contains a 64-bit set of flags for each
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page, indexed by PFN.
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The flags are (from fs/proc/proc_misc, above kpageflags_read):
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0. LOCKED
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1. ERROR
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2. REFERENCED
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3. UPTODATE
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4. DIRTY
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5. LRU
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6. ACTIVE
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7. SLAB
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8. WRITEBACK
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9. RECLAIM
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10. BUDDY
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Using pagemap to do something useful:
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The general procedure for using pagemap to find out about a process' memory
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usage goes like this:
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1. Read /proc/pid/maps to determine which parts of the memory space are
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mapped to what.
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2. Select the maps you are interested in -- all of them, or a particular
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library, or the stack or the heap, etc.
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3. Open /proc/pid/pagemap and seek to the pages you would like to examine.
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4. Read a u64 for each page from pagemap.
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5. Open /proc/kpagecount and/or /proc/kpageflags. For each PFN you just
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read, seek to that entry in the file, and read the data you want.
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For example, to find the "unique set size" (USS), which is the amount of
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memory that a process is using that is not shared with any other process,
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you can go through every map in the process, find the PFNs, look those up
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in kpagecount, and tally up the number of pages that are only referenced
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once.
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Other notes:
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Reading from any of the files will return -EINVAL if you are not starting
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the read on an 8-byte boundary (e.g., if you seeked an odd number of bytes
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into the file), or if the size of the read is not a multiple of 8 bytes.
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