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