861d2625b2
Our file_stream interface supports seek, but when we try to seek to arbitrary locations that are smaller than an aio-boundary (say, for instance, f->seek(4)), we will end up not being able to perform the read. We need to guarantee the reads are aligned, and will then present to the caller the buffer properly offset. Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
36 lines
1.2 KiB
C++
36 lines
1.2 KiB
C++
/*
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* Copyright (C) 2015 Cloudius Systems, Ltd.
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*/
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#include "fstream.hh"
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#include <new>
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#include <malloc.h>
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#include <algorithm>
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future<temporary_buffer<char>>
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file_data_source_impl::get() {
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// must align allocation for dma
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auto p = ::memalign(std::min<size_t>(_buffer_size, 4096), _buffer_size);
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if (!p) {
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throw std::bad_alloc();
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}
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auto q = static_cast<char*>(p);
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temporary_buffer<char> buf(q, _buffer_size, make_free_deleter(p));
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auto old_pos = _pos;
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// dma_read needs to be aligned. It doesn't have to be page-aligned,
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// though, and we could get away with something much smaller. However, if
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// we start reading in things outside page boundaries, we will end up with
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// various pages around, some of them with overlapping ranges. Those would
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// be very challenging to cache.
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old_pos &= ~4095;
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auto front = _pos - old_pos;
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_pos += _buffer_size - front;
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return _file.dma_read(old_pos, q, _buffer_size).then(
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[buf = std::move(buf), front] (size_t size) mutable {
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buf.trim(size);
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buf.trim_front(front);
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return make_ready_future<temporary_buffer<char>>(std::move(buf));
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});
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}
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